Depends entirely on what you plan to keep.
Dwarf ones you can keep with anything that won't eat them... dietary requirements about the same as a dwarf shrimp... although if you want them to colour nicer- veggie based foods supposedly are better than meat based.
I've never kept the larger ones because any small fish is likely to be attacked at night and it would be bye bye snails and shrimp. Any large or aggresive fish could potentially be a threat to any cray when it moults and is vulnerable.
Best to know what kind of cray you want. Do you want a big one? Do you want to keep other critters with them?
Do you plan to keep plants- some are more destructive than others... again dwarf ones are safe with plants.