Physics-wise, Lemmix can be considered equivalent to CustLemm (except for a very few obscure things, like pausing at the start to get some extra time, and the nuke percentage glitch).
If you're playing .dat levelpacks, you can basically be assured that they'll work in CustLemm/Lemmix, while you can't be certain whether the levels will be solvable in Lemmini.
For the record, Lemmix also offers action replay and saving replays, and savestates in addition.