No problems to get them to eat. The smallest one has been imported three weeks ago. It started eating live tubifex worms the day after arrival at the importing LFS. During the first week it started eating frozen bloodworms. My LFS will only give them to customers when they are eating well. This small ray is in very good condition.
I would never feed the flower rays with fish, because IMHO this is not their natural food even for the adult ones. They will get bloodworms and all available kind of mosquito nymphs, shrimps, gammarus, krill and other simillar invertebrates.
I prefer keeping the flowers alone without other rays species, especially not with black rays or motoros, because these species show an aggressive behaviour during feeding time compared with the gentle movements of flower rays.
Tiger Rays would be good tankmates.