Red Tail Shark

YoYo's can be fairly aggressive and defensive in their own right. I'd say they should be fine together. Just make sure to have plenty of hiding places and plants, wood, etc to break up the territory so they can stake claims.
 
I think they should be okay as long as the shark has its own retreat from the other fish. If they're already in the tank it should be fine to introduce the shark in, but don't introduce anything that can't fend for itself after the shark, because it will already have an established territory to deffend.
 
Give the shark a cave structure to hide in. It will almost surely become the shark's territory and keep the shark away from the other fish except at meal time when the shark will chase any fish he can away from the food. I've solved this by feeding sinking algae wafers to the shark and floating food to the rest of the fish.
 
Clown Loaches will outgrow a 55 gallon. Personally I like Rainbow Sharks. They look like the red tail just they have red fins also. Same personality and requirements though. Against other fish, the rainbows are slightly less agressive though they will chase other fish around every now and then.
 
Clowns are one of the few fish that almost always get suggested for going with RTBS. They'll get along fine and eventually outgrow a 55 gallon but this can take years by which time you can either upgrade or sell/give away the fish to someone who can house them. Just be mindful of stocking levels and not push them and all will be fine.
 
AquariaCentral.com