A black moor would be an excellent choice for your fish to have as a friend!
Please remember to quarantine your new fish first for a week or two to make sure you don't pass any diseases or parasites to your oranda.
A Black Moor will eat anything your oranda does since they are both goldfish. The sinking pellets would be a great staple diet with some frozen peas with the skin taken off, maybe some freeze dried shrimp or worms, even some chopped spinach or romaine lettuce or some zucchini can be given to them once in awhile as a treat.
As far as tank size I would say for the fancier goldfish you are planning on keeping that no more than 4 to the 46 gallon and 5 would probably live quite comfortably in the 55 gallon. Remember that overcrowding will cause extra work for you to keep the tank clean and you might end up having to do water changes more often than once a week.
Good luck with your new fish and please post a picture when you can!