Okay, well either of those tanks would be a great spot for crays.
Do you plan on only having one cray? If not, then I'd wait and add all crays at the same time. Otherwise, a lone cray will feel that his/her territory is being invaded, and likely try to get rid of its 'opposition.'
Also, it is typically recommended that you not put more than 1 cray per 10 gallons of water.
Neither of these "rules" are set in stone, and providing plenty of hiding spots/caves, etc., for your crays should help to curb a lot of the stress and aggression that could occur in an otherwise barren tank.
As for recommended fish to house with them...
Generally-speaking---stay away from other bottom-dwellers (no plecos or cories) and any fish that have long, flowing fins (no bettas or guppies).
Remember, there are other larger fish (from the cichlid family, for instance) that will dine on your cray, so keep that in mind too! Plenty of hiding spots should curb this tendency though.
Sorry...I could probably ramble on all day here...
Let us know more specifically about the stocking ideas you had for your tank(s), and I'm sure you'll get better feedback.
Big Vine