When sampling for your mimic the special attacks are always random. Though more times than not you will pull a sample with no specials, this can be frustrating to say the least. I had made a suggestion post about having a turn-in barrel much like the one for powerscrolls, so you could dump all the wasted vials and get a few fresh ones. If that suggestion was taking in or even looked at though is questionable. (i thought it was a great idea)
As for which mobs to pick for your mimic. I would -highly- suggest a mob that is mountable as bios can not be shrinked. These are sch mounts like horses, fire steeds, ridable beetles, ridgebacks, ostards, etc.