Base Idle Animation, Random Idle Animation (from a vast range), Base Idle Animation, LOOP

No, in the Variables tab. The variable type is Anim Sequence.

TBH, if you want a very straight forward setup, just add a Random Sequence Player node in the random idle state and assign your animations to it. Enable “Automatic Rule Based on etc…” on the transitions between idles and it will switch between states when the animation finishes.