That's quite a disparate list, so it might be hard to find a logical reason they are all working together. So it might be better for the leader to be the true shapeshifter monster, with a couple shapeshifter underlings but most underlings aren't shapeshifters but still have that flavour through the use of magic items/spells/abilities.

For example, having the main villain be a Rakshassa with one or two Succubus Lieutenants, and a bunch of lesser minions that are Fiend Warlocks whose Pact was either with the Rakshassa or the Raskshassa boss who is still in the Nine Hells. The warlocks use Mask of Many faces which still gives them that can be anyone at anytime feel.

You could also consider Druids for shapeshifters, an evil circle of moon druids who work with lycanthropes and a barbarian tribe to attack civilization. Maybe even evil Pixies who use Polymorph to shapechange their minions.