Thanks Michelle! I've heard that neons can be a bad mix with bettas - I took a couple precautions to try to eliminate any aggression, and so far they mostly leave each other alone. But I did know it was a risk. I got lucky, my betta is pretty mellow. She just likes to swim back and forth the length of my tank in peace (avoiding the air stones, of course). I also made sure she had a couple surface hiding spots (floating foam flowers that give her space to hide between the petals when I turn it upside down), and she was the last fish to go into the tank so she had no time to feel territorial in the tank alone.
Hopefully she'll do well when I switch the neons with a platy that's more her size.