It's intended for use with species that use the vanilla arm configuration.
If it doesn't, that probably means that that species either doesn't have 'swim' arm frames, or is using some other kind of arm configuration entirely.

for some reason it's not working for the slimes?

Ok thanks!

It's both, actually.
It's clientside in the sense that it won't break anything for anybody, but the poses will remain visible for all players on the server even after you remove it ^-^

Is this clientside? I wanted to make sure if I remove it, it wouldn't break anything even if a friend is subscribed to it.

saminamina eh eh waka waka eeh eh

Okay, I'll try it out and let you know, sorry for the question btw. Just been a while since I've been on the game lel.

I can't say for sure, but I think it should be compatible, as it doesn't add anything that isn't already in the game, and is purely cosmetic.

Not entirely sure how this stuff works, but is this compatible with servers? I'm betting it isn't, but want to double check.

Yes, there's no reason why it shouldn't be. It merely adds even more poses :)

Is it compatible with 'More Idle Poses'?

Hehe... modlet... hehe...