A lot will depend on what type of crayfish you have already. They are either Clarkii or Alenie. Clarkii are larger and more aggressive. In your 55, I would not put over 5 animals and that is pushing it. I would be afraid that CPO-Dwarfs may end up an expensive meal for the blue crays. If you really want dwarfs, I would place a few sections of PVC, say 4" in length in that are of a small diameter keeping big crays out and dwarfs in. If you go with more larger crays, white Clarkii are cool and they also have red and orange.
I don't know what you feed them now, but I feed mine each a half of blanch pea everyother day. They also get a piece of shrimp pellet every week. Crayfish are pigs if you let them be. Keep in mind, they are constantly sifting and tasting the substraight. They eat a lot of bio layer you don't see. Mine also love to eat snails. I have an ongoing supply of pond, bladder and ramshorn snails in other tanks. These also help them maintain a good amount of calcium as they eat them shell and all. If you toss in say ten, some are bound to lay eggs and there is an ongoing food source.
Just incase you didn't know Cherax and other AU crayfish can not be mixed with American crayfish. The plauge zaps them everytime. Our crayfish don't get the disease.
