I would go with 6 in a 55, if you plan on doing it as a display/show tank. My 65, the only show tank I have running, has 7 angels, it works out well, nice balance between growth & seeing just enough fish.
If you arrange the decos right, you can minimize problems if & when they pair up. Try to use some tall plants as a divider, taller driftwood as well.
I have a couple of 55s that are more on the line of what John Nicholson is talking about, but that is how a breeding setup works. Bare bottom, homemade racks, filters & tops, crammed with fish, really not showy at all. Unless your a breeder, I guess.
Tolak