I removed the list in my previous post as the koans AK and E disproved at least some of my rules there.

I doubt that symmetry is really the key to this rule, since GRAY has buddha-nature but GRUY does not.

My thoughts actually went more in the direction of rules like "AKHTBN if if contains two vertically symmetric letters that have distance at least 10 in the alphabet."

Some new koans:
AA
AAA
AAAA
AAAAA
AAAAAA
GRAYL
ABL
TRYL
REDL

@Proxima: I noticed two typos in your list: ABNZ should be ABNS and NEGOTIONABLE should be NEGOTIATIONABLE.

Three more koans:
QJRGLPFAB
ABZ
ABNS

YGRA
AYGR
GARY
NEGOTIABLEQ

Here my impressions of the new/revised levels:

Tricky 10: Two solutions.
Tricky 27: It can be beaten with only one builder as well . But I agree on the "good level" part of namida's remark.
Tricky 28: Solvable and not as hard as namida's remark would suggest. Tricky 29 is far harder...
Taxing 1: Likely the same backroute as namida's.
Taxing 2: Huh... even the solution I thought was intended doesn't work any more, because I can't turn around the lemmings which are going to the gap on the left. But with a little glitching the level becomes quite simple.
Taxing 4: The now deadly fall makes the level much more interesting, but the variety of skills still creates many solutions. Attached only one no-builder solution, but if you want, I can record several more.
Taxing 6: I agree with namida's remark.
Taxing 18: My solution still works.
Taxing 22: This one turned now into an excellent level!
Taxing 25: Not sure whether this is intended?
Taxing 26: Much more interesting than in V1.
Taxing 27: Good chance that this is a backroute as well.
Taxing 28: Not much difference to before.
Mayhem 4: The level got somewhat harder, though I am still not sure about having found the intended solution.
Mayhem 7: Solution with only 7 builders (because when planning my solution, I was somehow under the impression that I had only so many builders )
Mayhem 8: My solution is exactly the same as before. I don't even know what has changed?
Mayhem 10: Good level.
Mayhem 11: Absolutely excellent level !
Mayhem 12: 99% saved.
Mayhem 13: Good level, though I already knew this trick from the Lix community level pack.
Mayhem 14: I too can save 96% without much trouble.
Mayhem 15: The time limit is very tight and the level is solvable with 5 skills of each (though my solution uses 6 builders for convenience).
Mayhem 16: My backroute still works. And you forgot this level when setting your one-way arrows to "only on terrain".
Mayhem 17: 100% possible.
Mayhem 18: Another absolutely excellent level !
Mayhem 19: I still have one basher left over. But it already was and still remains a very good level!
Mayhem 24: This is the least endearing change you made from V1 to V2. The method is the same as in the first appearence of this level, but only much much more fiddly.
Mayhem 26: The third absolutely excellent level you added !
Mayhem 27: My solution is not much different from my previous one...
Mayhem 29: Pretty sure, this is not quite intended.
Mayhem 30: Far better than V1, though I still have a few spare skills.

Fierce 8: I disagree with Fierce 8 not being "too hard". Up to now I checked quite a few approaches, but all of them broke down at some point and none of them would have saved the required 92% anyway... As I will never ever read hints to levels I haven't solved yet, you may have to wait some more before you get a solution for this level (at least from me).

Fierce 11: It's a pity that you decided not to fix this level. Although it's of course your decision whether to stay with the current version or not: Have you considered going back to the level in V1? In my opinion, the current version features some solutions that are relatively standard and uninteresting, like my sol. 4.

Fierce 13: Finally solved this one - great level . The solution's elegance was a pleasant surprise as I expected that I had to use

some trick using the new basher-mechanics.

Fierce 14: I can save 90%, so it's likely another backroute...

Fierce 11 still causes problems... On the positive side, contrary to Fierce 15, I see hardly any way to modify my solution. So there is a good chance, that your next fix will actually remove my way to solve the level.

Thanks for creating these excellent levels !

Fun 30: A great level for finishing Fun.
Tricky 4: Another great level, but too hard for its position.
Tricky 6: A level that feels like it could become a great puzzle. Unfortunately it has an easy solution saving 100%.
Tricky 12: I agree with namida about the bomber placement.
Tricky 19: Nice little level - assuming you reduce the skill set... At the moment it feels very much like a Fun level.
Tricky 21: An excellent level. Far too hard for Tricky though, because one has so many possible approaches to this level. See my comment to Taxing 20 as well.
Tricky 26: I don't need the pixel-precision of namida, but my solution is nevertheless nothing you should have in Tricky. And please make it clear to the player that the direct drom from the middle platform is survivable (it's only 61 pixels). When I first reached this level, I spent quite some time planning how to get the lemmings down, when this is trivial and automatic.
Tricky 27: This is another trick that is very well-known. Contrary to namida and geoo, this is not one my favourite Tricky levels (though it's definitely not bad).
Tricky 30: A very long level and I need every of the 20 builders (and during my first attempt I was even 1 builder short and had to do almost everything again). Together with Mayhem 28 my least favourite.
Taxing 2: A very good level. But you should raise the save requirements to remove the backroute I attached (assuming it is indeed a backroute).
Taxing 7: very generous on the save requirements.
Taxing 9: A good level - for early Tricky. It is easily solvable with only 2 climbers, 2 floaters, 1 blocker and 1 miner.
Taxing 12: One of the very few builder-heavy levels I ever encountered that I really liked.
Taxing 13: Good level, even if it's not really hard to save 90%.
Taxing 14: I can't agree with namida's remark - I had over 1 minute left.
Taxing 15: Way too easy with a great deal more skills than needed.
Taxing 17: I guess I found the same backroute as namida.
Taxing 19: Another very good level that is easier than placed. I used only 4 skills.
Taxing 20: This level is easier than Tricky 21 and I don't think the beginning is that great...
Taxing 22: Though not completely original, it's always nice to see such a level again.
Taxing 23: Absolutely unique and excellent level!
Taxing 24: Contrary to namida, timing is an issue with my solution. Nevertheless my basher usage is a strong indicator that I found a backroute.
Taxing 25: A very very good level.
Taxing 26: Sorry, but this level is straight-forward and not interesting at all.
Taxing 27: Feels like a slight variation of the intended solution.
Taxing 29: A very good idea and my solution doesn't rely on the glitchy behaviour namida mentiones.
Mayhem 1: Again a direct drop works, though it is not necessary. Whenever I want to remove this possibility of a direct drop, I try to place a small terrain piece behind the exit directly over its trigger area. Then the falling lemmings usually splat (as they should) instead of jumping into the exit.
Mayhem 2: One of the three best levels in your pack! Even if it didn't take me long to work out the solution.
Mayhem 4: Fortunately one can save 90% and thus reduce the amount of bomber timing.
Mayhem 5: So much unused terrain about... I really wonder whether my solution is intended?
Mayhem 8: Way too easy for Mayhem and more or less staight-forward.
Mayhem 9: Even if your level isn't the first that features this strategy, it was very enjoyable.
Mayhem 10: Not a fan of this one.
Mayhem 14: An absolutely excellent level ! But please remove the backroute I attached as a second solution.
Mayhem 15: And again a very good idea, even though the first half was pretty uninteresting.
Mayhem 17: I already solved the first version of this level in this way.
Mayhem 19: Digger placement was slightly annoying, but otherwise a good level.
Mayhem 20: I likely backrouted this one.
Mayhem 21: ...and another likely backroute.
Mayhem 22: Together with Mayhem 2 and 14 the best level! I never needed pixel-precise skill assignment anywhere.
Mayhem 23: Another backroute.
Mayhem 25: Apart from a very precise bomber placement, this level was much fun, though easier than its placement.
Mayhem 26: A few good ideas, but too heavy on builders.
Mayhem 27: An excellent mulit-tasking level! As with Tricky 26, please make sure that the player doesn't mistake the drop on the very right for a splatting fall.
Mayhem 30: I guess this is more or less the same relatively easy solution namida already found.