i have convicts and about 4 inch blue lobsters (actually are crayfish). the crayfish can hold their own. the convicts are quite defensive and did try to eat the crayfishes when i first got them, but after realising that the crays had pincers they soon stopped. aslong as you have hiding places then both are fine. none of my fishes have been hurt by the crays because they are aware that these lil buggers can attack. however my fishes are 4 inches max as opposed to your 6 inches.
obviously i may be lucky and also your fishes are huge, but i think that if you have hiding places then they should be okay. infact i think they will be aboslutely fine. crayfish are quite elusive until they get bigger i have noticed.
once they are about 4 inches they explore in daylight so thats nice to see.