GuppyGoddess
Fish Herder
Edited - am going to try three girl bettas.
Just wondering - why not cycle the tank first? Much safer for the fish and much less stressful for you.
As for tank-mates - females can be as moody as the boys so anything you try she might not accept. In 10 gallons (US gallons, right?), I'm not sure what you could have that would be OK without friends and have a big personality. Normally I'd recommend pygmy corys, shrimp or snails, and maybe some green neon tetras (although I'm not sure if they need a bigger tank). Corys are actually pretty awesome - I didn't like them until I actualy got some and now I'm a total convert. They do need to be in groups of about 5 though. Some people have sucess keeping them in threes.
There might also be some wild bettas that are actually OK to have a couple of, but I don't know if your betta splendens will fight them. I'm not really the expert.
EDIT - be careful with only three. I only have three in my community tank, but that's a short term measure and they will soon be in a larger group in a different tank. Sometimes in groups of three two gang up on one of them and you can end up with dead fish.