+1
There are a lot of fish possibilities to go into the 55 gallon. "Tetras" is a very wide spectrum of fish. There are fish like neons, but there are also some very large varieties as well, like silver dollars.
You could go for some rainbowfish. If it is shorter, you could go with dwarf rainbows like the Praecox. If it is a long tank, boesemani rainbows are a cool fish, lots of color. (btw, the rainbow has more to do with the shape of the fish than the color of the fish.

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Praecox rainbowfish
Boesemani rainbowfish
You could consider rasboras or barbs. (Be careful with the barbs though, they can get a bit nippy, especially with the corys, if you get the wrong ones. Tiger barbs are some of the nippiest, and best avoided with corys, IMHO.) Five banded barbs and cherry barbs are peaceful fish that would go well in your aquarium. These are some smaller fish, so they may not be to your liking as much, but they would go in a shorter 55 or a long 55 very well.
Five banded barb
Cherry barbs
All the above fish are shoalers, so they will need to be kept in groups of no less than 6.
FInally, there are apistogrammas (dwarf cichlids). These are similar to the rams, but work best as a male and several females rather than a pair. (I'm not sure how they would work with the ram , however. Hopefully someone else can speak on that.