I would think perhaps you could introduce a high-level fish... it would stay out of the crayfishes way. But you're looking at limited space since it's a 5
gal.
As for the habitat, if it were me, I'd do either black or white sand (sand is so much easier, to me- and crayfish will bury in it) about 1 1/2 to 2 inches thick.
Then I'd do plants, probably false so they wont get torn up or easily uprooted. I'd also buy either rocks or cave type decor from the
LFS to create hiding places. Not so much that you'd never see the guy, though!
Keeping one is a good choice, because they are territorial. They can get up to 5'' in length, as well, so two would be crowded.
They like the water to be within 68-85 degrees and the pH can be anywhere from 6.5-8.0, so they're not picky. They're omnivorous and will eat sinking pellet food, flake food, and algae.