You may want to do a little research about those tetras. There is a reason I got rid of mine. If kept in large groups they will harass each other to the point of exhaustion. If kept in too small of a school they will harrass other fish.
Edit: sorry shoulda mentioned the real name... Lamp eye tetras.
I'd get two more red eye-s... i got three right now and are ok but two more would definately take the load of the ones that get tired. My guppies are fine with them and my danios are as well... Just for each others sake id have 5. As far as other fish.. definately get a catfish of somekind. Got to try to fill the tank with fish that take up other parts of the aquarium now. With red eyes in a ten gallon.. they are pretty much gonna be your main school. You could try a molly or two.. preferably dwarf mollies of some type. you can add color this way.. Or even some guppies. I dont have any fin nippers in those tetras what so ever. good luck
He doesn't have room for anything else. This is why I said research. They get almost 3 inches long. They are one of the largest tetras I have ever seen and have social issues. Go figure a schooling fish that is at school to pick a fight.
theyll grow to that size in a bigger tank. until then he can get 2 more instead of another kind of fish. would rather him add 2 more than another kind. but a catfish wont get in their way. And if their social behaviou is addressed with plenty of each other to dither them ... he can stretch for 2 mollies or guppies for color. If youre on top of water quality... you can do it.... if your a bit on the lazy side... stick with 5 red eyes total.
Better to start looking for a larger tank now, or trade your red-eyes in for something more 10 gallon compatible. I've kept them without any problems but they are known for nippiness and you'd pretty much max out a 10 with one or two more.