1. Release Rate Practice: Odd title. Sure, it's convenient to raise the RR to 99 to speed things up, but it's not necessary, and that leaves most of what you can do with the release rate unexplored.
2-4: Okay skill tutorials but the 100% requirement is a bit harsh. I suppose that underlines the fact that this is a NeoLemmix pack and you expect the player to make use of features such as undo, splat ruler and skill shadows.
6-7: Much trickier than expected for tutorials, with some precise placement needed in 6, and timing / crowd control tricks in 7.
9. Building Test: The first of several levels in this rank with a lot of building. I know it's much easier in NL with the ten-second skip, but it's still one of the more tedious ways of getting from point A to point B. I used an advanced trick for containing the crowd; not sure what was intended here.
10. Treat Earth Well: For the first non-tutorial level, this would be tough for new players, with the trick of working with the last lemming, not much leeway with skills,
and the first trap.
11. Cold Night: Much easier.
12. Extremely Tough Level: ...it really wasn't all that bad
but again, the trick would be hard for new players, making me wonder whether the level really belongs this early.
13. Four Temples: I like the concept, but would have preferred more variety among the sub-levels instead of mostly building.
15. Old Palace: Dense and chaotic. No idea if there was a different intended route, but I'm sure there are lots of ways to do it. However, this is fairly demanding for an early level, especially with the 100% requirement.
17. Strange Place: Yay, a break from building and a nice puzzle with a small number of skills. I saved 100% when (for once) the level is lose-5, so I'm not sure if what I did was unintended or you just built in some leeway. Could add a talisman.
19. Very Dangerous Trail: This was horrible. There are so many falls that are only just above splat height, and the limited skills meant I couldn't take the precautions I wanted, to make them safe by a comfortable margin, so I had to do a lot of testing and rewinding. (The splat ruler doesn't help much when a fall is borderline, unless you're very good at reading it.)
20. Search in Snow: NL reveals the hidden exit easily, so this is just a large "get through a varied terrain" level. At least it's a pretty fun one, with decent leeway and lots of room to choose your own route.