The original clones from Eric's site may not work because the "Original Lemmings" clone only has the graphics sets for that game (and not the ONML graphics sets which some CustLemm levels may use), and similarly for ONML clone (which in addition to missing sets, the numbering scheme of the sets is not compatible with CustLemm's). At some point I think namida posted an updated version of LemmixPlayer that has all 9 graphics sets included (and numbered just like CustLemm), so I think in principle you can use that one to play CustLemm levels if you can find where he posted it.
It is still of note that there are some minor game-mechanics difference between CustLemm and some of the other games, so some levels may not play out perfectly on the clones:
- on original Lemmings, the entrance ordering for 2 entrances is ABBA rather than ABAB like ONML and CustLemm. Don't know which ordering namida's modified version uses.
- on original Lemmings, the lemmings come out of entrance trapdoors one more pixel to the left compared with ONML and CustLemm, again not sure how that goes on namida's version.
- On CustLemm only, the maximum safe-fall distance is 3 pixels more than original Lemmings and ONML, so for example in CustLemm the lemmings can safely fall off the end of the platform in "We All Fall Down". No idea what this is like on namida's version.
Somewhere on either this forum or Eric's site are instructions on how to use the LemmixPlayer clones to play custom levels, I don't have time now to look that up though.
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I personally just use the Lemmix editor to play CustLemm levels, but I can see why using a more game-like program to play the levels can be preferred. You can of course also just use DOSBox to run CustLemm for real.