I think yes, more bettas will overstock the 10 gallon. Bettas are territorial fish. If you have two bettas in a 10 gallon that live together peacefully, I applaud you, good sir. Some people do not even recommend more than one betta in a 20 gallon.
Compatibility with bettas come down to some basic ideas.
Do the fish look anything like a betta? Bettas will flare and attack anything that could be potential competition for mates and territory. Guppies, although are clearly not bettas, are generally incompatible with bettas due to their flashy fins, and flamboyant colors. Mollies and platies might encounter similar issues with bettas.
Will the fish out-compete the betta for food? Bettas are slow eaters.
Do the fish require different tank conditions than bettas? Bettas like relatively still water, are of course tropical, and appreciate freshwater.
Some fish you could consider as tank mates for your bettas are short-finned tetras. However, as they are schooling fish, you may hit stocking problems in a 10 gallon.
That's my two cents. Perhaps someone with more experience with bettas can also weigh in.
EDIT: Have you considered cories? I believe bettas leave them quite alone.