You really really really want to approach tank stocking as a comprehensive plan, not a one at a time thing. You have lots of options for a 55, if you plan the whole list you can avoid a lot of issues.
A big issue is aggression, but there are a few other considerations. Aggression level is important when you stock a tank because you can avoid some problems by adding fish in order from least to most "mean." That gives shy fish a chance to settle in and claim their space, and when you add meaner fish later they accept the shy ones. If you go the other way, and let a mean fish claim the whole tank, it will often kill any less-tough fish you try to add after. While there is individual temperament, different fish species are fairly predictable in their aggression levels. Some clowns are fairly mellow, while certain species can be very mean.
When I had a 55 I had a pretty happy mix of fish. I based my list on where the fish like to live in the tank, so I didn't have territory issues; and a variety of sizes and locations looks better. I basically had:
2 fish that like to live down on the sand
- watchman goby, engineer goby, etc.
1 fish that liked to prowl around on the rocks
- starry blenny and lawnmower blenny are good because they also eat algae,
3 small fish, and 1 larger that swim around in the water
- small: clown fish, firefish, chromis, Midas blenny, etc. my fave is my chalk bass
- large: I love my melanurus wrasse, but there other kinds too.
I think it would be hard to get 7 fish to share the tank if you just put them in one at a time, but if you plan for it it's doable even with these wild animals.
