In the wild, fish school for protection. In our tanks, they will usually school pretty much when they are first put in the tank as they are still in protection mode. Once they realize there isn't a predator in the tank, they usually split up and go their separate ways. You will almost never see more than a few of them together except at feeding time.
If you only plan to have tetras or other small community fish, I don't think you really have to worry about nippers. Red-eyes are very hardy as tetras go and are a good one for a new tank. When I set up my first tank, I cycled it with red-eyes (before I knew about fishless cycling but at least with an old filter pack from my sons tank). I have black phantoms in my 75 gallon (only 3 left now though) and they are nice fish too. It's also very easy to sex them too as the male and females have very different coloration.
I will second what Colon said about the black substrate. In actuallity, I think it really brings out the color of any fish but the reds in particular. I think neons, cardinals, rummynose, bleeding hearts and bloodfins would all look great with black substrate (I use tahitian Moon Sand in all of my tanks except the 75 gallon which has Eco-Complete which is also black).
And you should be fine with 10 & 10 on the small tetras provided you have good filtration and a good maintanence plan. Yes, it's over the 1 inch per gallon guide but they are light waste producers. Just don't go overboard to start with, even if you do a fishless cycle. I would probably start with 5 & 5 and let the tank matrer a little before upping the numbers.
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