Full size crayfish will absolutely destroy a betta. Dwarf crays, however, are safe to be kept with bettas. Just make sure you provide some small caves that the betta can't get into or it could easily eat them when they molt. How many dwarf crays you could keep is a function of how large your tank is.
125 - BGK, chanchito cichlid, pictus cats, silver dollars, palmas bichir
125 - cichlids (severums, bolivian rams, chocolate), rainbows ( turquoise, red), loaches (angelicus, zebra, kuhli and horseface), plecos (BN, RL and clown), denison barbs, tiretrack eel, pearl gouramis, betta
90 - Congo tetras, african knife, upside down cats, spotted ctenopoma, kribensis, delhezzi bichir
2.5 - betta