I was thinking the round tail paradise fish, they like the same basic ranges as the spendens and are hardy in cooler water as well. And they top out at 3 inches for the males. Should have been specific, ,since regulars I have sen one a whopping 5 once, instead of four. Blacks are 5 inch fish. Waay too large.
Neons annoy bettas, so it depends on the betta if he decides to actually hunt them down or play along. Mine plays tag half the day with the penguins. They are all larger than he is now, but he doesn't care. But that is a 40 gallon, they speed around to end up behind him before he has gone 4 inches... ^_^ .
Shrimp very often become snacks.
Most frogs do allright.