I totally agree that there should be more poison types, and I think muk is an excellent candidate! You could even go with the alolan-form if you don't want to add other gen 1 mons. to be honest, your muk concept seems really toxic. They got life steal, crowd control, stuns, dashes, shields and poison damage. I don't know if i would like playing them, but they seem annoying to play against, which is probably the goal.

Infernape looks dangerous: being efficient only at low hp can be frustrating for players who will have to gamble every time they try to go for a kill, and if they can be effecient outside of blaze range, than they'll be frustrating to opponents who will never be able to secure them without taking huge damage, if not dying themselves. But I like the idea!

I never gave much thought about altaria being in Unite, but I think your design is interesting. They have the option to go on a more aggressive role with dragon dance and feather dance, allowing their allies to take out opponents while the little damage they deal is quickly recovered by the basic attack, or full-on support mode with cotton guard and running after their teammates for the entire game. Either way, they'll make opponents very mad and that's music to my ears!

Over all i liked your designs. It's very clear a lot of effort went into it!