I had to look up the recommended tank size, but according to the link at the end of this post 10 gal is the smallest per 6-7 inch cray.
So, all your tanks are able to hold a crayfish. However, I agree that the 56 gal isn't an option due to the plants (crayfish WILL uproot, redecorate, and essentially drive you crazy in a planted aquarium. However, the fish and shrimp might be okay with them but I still wouldn't risk it
)
The 20 gal isn't an option as both crabs and crays "need" a species tank even if you could get by in a community. However, the better reason is that RCC need a brackish water (~1.008-1.012 specific gravity) tank to survive long-term, though some have reported success in a purely freshwater aquarium. These crabs usually only last around six months in pure FW though.
*crays are FW, therefore incompatible with RCC
The 10 gal is your best bet for the cray, though be sure it doesn't have the cichlid. I had a blue (electric?) cichlid once and EVERYTHING we put in its tank was food except the fish it grew up with. My parents bought that tank used, else we never would've had him.
To be fair, though, it's more enjoyable to see an animal move around its tank, so I really wouldn't keep it in anything under a 20 Long with fast moving surface fish such as guppies, and the guppies breed prolifically (they're called the millions fish for very good reason) and may feed the cray on occasion (if you don't mind the natural life cycles being played out in your tank).
just feed vegetable matter (blanched zucchini, cucumber, kale, romaine lettuce, etc, just stay away from iceberg lettuce because it has no nutritional value) and it'll be entertaining to watch the cray eat the veggies whilst the guppies dart in for a bite while trying not to get eaten
~GreenGo, future Aquarium Population Ecologist with an emphasis in Sustainability and a preference for crustaceans (FW crabs and shrimps; crays are illegal in AZ, but that's how I came upon crabs which IMO are much more interesting)
http://bluecrayfish.com/north_american_crayfish_care_she.htm
Keep us posted!
Sent from my Nexus 9 using Aquarium Advice mobile app