Your single cae should actually do ok in the 55 gallon. Make sure there are at least 2 (3 is better) caves that he can hide in. Feed him the sinking pellets that are made for cories. At 2 inches, he is actually about teenager size. The main reasons they attack other fish are:
1. defending territory - not enough space - not enough hidey holes
One cae needs at least 55 gallons - more is always better. They also need three caves that they can move between. They will generally choose one favorite, but will defend 2 or 3 spots from all other fish. Another reason why they need a lot of space.
2. not enough food
They are voracious eaters and most causes of them going after other fish is because they are scavengers and are getting hungry from not enough food. They also prefer meatier foods as they get older. The sinking pellets for bottom feeding scavengers are a good choice. One cae will/can eat 3 or more of these pellets a night as well as keeping the algae under control.
Cae's are basically huge shark type fish in a small body. They need lots of room and lots of food. If they are given what they need, they do not bother other fish. I have three cae's for my pond. They do not bother my plecos,
goldfish or mosquito fish at all. They have lots of places to play hide and seek from each other and lots of space to swim around. They also have lots of algae and pond snails to feed on as well as the sinking pellets I drop in.
even in bigger tanks, they crave or seem to crave , slime coat as adults..I had 2 in my 75 gal for a few years...and then had no clue why all my angels had all these weird spots on them...and then saw one on an angel one day and found out from FL and other sites, that CAE's as adults, seem to need the slime coat like cats crave catnip