55 G New Tank
By all means, use your own judgement, but your tank really hasn't been running very long, so the water conditions still need time to settle. You have a nice number of fish, so I'd let the tank run for several more weeks before adding more. You want to make sure there are enough bacteria to handle the current amount of fish waste.
If you want to work on your tank, then I suggest planting some more plants next time you do your large water change. Some good ones are varieties of Hygrophila, Pennywort and Hornwort. Anacharis is nice if you have good lighting, but I'd wait to add fish.
I have several 55 G tanks. They're my favs. I like to experiment with plants and fertilizers, so my tanks look different than most. Attached are a couple of pics.
Above everything else, have fun!
"Fear not, my young apprentice. Just change the tank water."