I don't recall the exact Lemmix physics to set, but Custlemm physics sound adequate. In DOS L1 physics, the level Don't Follow Me is not solvable due to the lower splat height in DOS L1.
And what's the splat height under Genesis?
Actually, if I recall, it's not a matter of different splat height per se. Instead, the difference stems from Genesis not having the "lower" splat height of a miner as you do in DOS L1. The short summary is that in DOS, (roughly speaking) a miner transitioning to a faller cannot survive as high a fall as a normal walker transitioning to faller. Whereas I don't think this difference exists in Genesis. But otherwise I
think the splat height is identical in both versions (but it's been a long while since I last checked).
Also, when I ported the Genesis levels over to DOS, I distinctly remember that you
can still solve "Don't Follow Me" as-is using the normal DOS splat height--you just have to make some big RR changes that are not possible in Genesis (because Genesis doesn't allow changing RR when paused).
Also, which Dos L1: the CD version or the floppy version or whatever? Because there's a difference. If you didn't know already, CD-Dos version of Lemmings has a splat height of 66 pixels while under floppy-Dos, it's the classical 63 pixels.
Almost all mentions of L1 refers to the normal version. I practically consider the alleged CD version with different splat-height non-existent, as the last time someone claims to have it and ripped the music, he never made available the game files despite I asking.