I'd just like to make a little suggestion - this is based on how I did all of my large lemmings packs apart from the very first Lemmings Plus on cheapo.
Rather than work on one difficulty rank at a time, just make levels (whether hard or easy), then sort out which difficulty you think they fit into. When you make a really hard one, put it into a high difficulty. When you make an easy one (assuming it's fun - if it's a boring level, you may want to redesign it or scrap it altogether), put it in an easy one. If you're doing repeats as well, consider whether levels you've made earlier could have a harder solution. Finally, if you're sick of making levels, do a few easier versions. Definately, do not place repeats next to each other (some of your levels so far do this, it really kills the fun of playing if you have almost the same level twice in a row).
In general though, if you're doing repeats (even if they're not exact repeats, such as is the case with the two Just A Minutes in the original game), don't overdo it. We All Fall Down is overkill, it's exactly the same thing repeatedly. Try to do repeats where the solution to the repeat isn't too obvious from the easy version (for example, you almost certianly will not solve Mayhem 13 anything like the way you solved Fun 26, and you almost certianly will not solve PSYCHO 13 anything like the way you solved Medi 1). Of course, in some cases, it may just be harder to pull off, or require a couple of extra tricks, and this is not always a bad thing if done well (for example, Tricky 30 has the same basic solution as Fun 14 but adds an interesting twist to it by not giving bashers, and the same is true for PSYCHO 25 and Wimpy 30). In short - repeats work well if they're done well, but are a plain nuisance if they're not, so if you're using them, take care.
It also helps keep it similar to the original if you throw in a harder level among easier ones sometimes, as well as an easier one among the hard ones sometimes. For examples of this in the original game, look at Tricky 19 (hard among easy) and Mayhem 25 (easy among hard). For examples of this in Lemmings Plus DOS Project, look at Medi 6 (hard among easy) and PSYCHO 14 (easy among hard).
Thanks to Lemmix's "LookForLVLFiles" switch, this is quite practical to do.