I would do 4-5 of the same kind. They kind of school with their own kind. Before I knew I got 2 panda corys and 2 albino corys. So far they seem to be pretty good with each other but all recommendations say at least 4 better 5 or more and the same kind. I like my panda's. They have great personality. I also liked the peppered cory. Some get bigger then others. I have mine which are a fairly smaller type in a 20g so I think you should be fine with 5 in a 55g. When I move my 20g stock into my 55g in about a year I'll probably add another 2 of each kind that I have already. I think the emerald kind get a bit bigger then the ones I've mentioned. Not sure what other kinds are out there but this is what I see in my LFS often.
Good luck.
Good luck.
). Do make sure you get them plenty of food though(sinking pellets are the best), most of the time they don't get enough in a community tank.