Tank mates

As just mentioned, I am starting a new tank. It currently has 3 smallish
Lemon Tetras.
1) How active should they be? Thus far they are not hiding but are pretty
passively slowly moving around the tank in a group. I don't know if this is
their nature or it's just because they are new and only 3 of em. They do
seem to do a lot of start-stop short swim movements, which I havent decided
if I like or not.
2) Could you give me some suggestions about what could go with them. I
know this is a small tank but I was kinda hoping to have two species in
there anyway. I know opinions and experiences vary but I thought you might
have some suggestions about some other fish that might be alright with just
3 or 4. Returning the Lemon's is not outta the question. I already have
Buenos Aires, Platys, Mollys, and Danios. I was thinking of perhaps Serpae,
Harlequins, Glass Fish and Hatchets or Cherry Barbs as possible mates. No
not all at the same time Neons not outta the question but I'm not sure if
it would be good or bad that my background is black, which I really like in
this tank. If you're thinking you could have _ and _ but only if you return
the lemon tetras, that's a possibility, although the wife DID pick them out.
Thanxx
Bill