1) If a merged slime creature affected by "teleport other", then it should split apart into regular sized slimes, and each gets a timeout assigned independently, so they won't (necessarily) all disappear at once. (They'd have to be unable to merge while still unstable)
2) If a merged slime creature is polymorphed it should split apart into regular sized slimes, each of which gets polymorphed into something new.
Even if slimes are split when teleported, they'd all get the same teleportation timeout. I think it makes a bit more sense to disrupt the slime when it actually teleports rather than before. I agree that two slimes with different teleportation duration should be unable to merge.
Making polymorph turn a single slime into a random number of monsters (depending on size or hd or whatever the counter is) sounds fun.
Right now if you teleport a merged slime it will start splitting after its foe_memory runs out (it starts wandering at that point). Rather than making teleport explicitly split slimes I'd rather have slime creatures start wandering right away so that they split on their own. In fact maybe it would make sense to have all mindless creatures start wandering after a teleport (they lose track of you because they are mindless).