It may be easier to combine similar enemies into a single sprite. Say, all blue enemies into blueEnemy sprite, all red into redEnemy sprite. This way you can create a blueEnemy instance and then pick a random animation for it.
Alternatively, you can create a random enemy using "Create by name" action. For example:
Create object by name: choose("blueSkeleton", "blueZombie", "blueGoblin")