Currently, whether a pack uses regular lemmings or the Xmas-y lemmings is determined by the pack as a whole, specifically which lemming graphics it contains.
I'm wondering how people feel about the idea of instead making this a per-graphic-set thing - this way, in a large pack, the Xmas levels could have the Xmas-y lemmings, while the normal levels have regular ones. It would also eliminate the need, where triggered objects (eg. teleporters, traps) are in use, to have two different versions of the graphic set depending on which one the pack uses.