Well the theory with the odd number with tangs is that they won't pick on each other if they have enough room. But if you have three tangs in a 55 I can garantee there would be problems, I would suggest only one for a 55 gal. Actually only one tang is supposed to be in at least a 75 gal, but IMO a 55 is fine. I like tangs a lot myself and wish I could get one but my tank is not large enough.
If your looking for color then a perfect fish would be a royal gramma, they stay pretty small, a good community fish and are quite hardy. Do you have any other fish in the tank other than the tang? If not then like in your list a flame (fire) angel is a good choice, they are one of the most hardiest of the pygmy angels and very pretty. Have you ever considered clownfish? These are very colorful and I think have the best personality and are a very passive fish, at least most species of clown are.
Also another fish that is quite interesting looking is the firefish, they are a bit shy but very cool. There are red fierfish witch are red and white then there are purple firefish, these a very pretty but cost a little bit more than the regular firefish but they are worth the extra money. I hope this helped you.