Danios need a group of 5 and at least a 20 gallon so no, a pair in a 10 would not work. Maybe consider honey gouramies - they are more difficult but need less space. Alternatively, maybe you could try a pair of rams (neotropical dwarf cichlids). Again, these are not easy but can be bred in a 10 gallon and, unlike the danios or gouramies, the parents are good with their fry. If you can get your hands on american-flag fish, these aren't too difficult to breed either and it should be quite possible in a 10 gallon. Just keep in mind that the fish I just suggested are NOT easy but they WILL breed in a 10 gallon and should be quite easy to sell for a good price to your LFS. I suggest you research these species more if you are interested though first so you can see what each would involve. You could also try certain shell-dwelling cichlids like n. multifasciatus in a 10 gallon. they are easy to breed (because you don't realy need to do anything) but these fish do have some specific requirements as they need snail shells to live in and come from african rift lakes so preffer certain water parameters. These would be sure to sell well though. I think cories might also be a possibility but you'll need to do some more research into this first.