Anyone still looking for this:
Use montages. Set the montage bone group, have the random idle animations as montages, and in the anim BP track time spent speed >= 5 (thats what i use, i dont like to use 0), when time = allowedIdleTime: Use a select node, random float between # of Idle animation variations as the index, animation montages as the input, selected random animation as the output, voila.
Hope you understand, im about to set something exactly like this up so I’ll repost shortly with a pic