People feed their octos different sorts of shrimp, as long as they're saltwater. Most octos accept live and thawed frozen shrimp. I used common shore shrimp for the live shrimp and raided the best seafood market for the freshest and best looking shrimp - pink shrimp from the Florida keys, Gulf shrimp of varying sizes. If you are near the ocean, you can even buy bait shrimp.
You miight be able to keep some sorts of crabs and shrimp together, but not really the fiddler crabs because they need to be out of the water a fair amount.
Nancy