I currently have two female bettas that i would like to get into a 10 gl tank. I read before that you should have at least three, but if any has ever done this before how many would you suggest? Also, my girls are pretty big eaters so i'm not too worried about maybe adding a couple other fish to the mix, any suggestions? I was thinking to changing to an under the gravel filter, which i've never used before...

If they don't get along, the tank is big enough that i could just divide it, but that too is something i've never done before.

Some betta females get along fine while others will fight. I've only had one female betta who, unfortunately, didn't live long. You won't know how they get along until you try. If they fight too much, they'll need to be separated. Bettas can get along well with short finnned danios. The danios are too fast for the bettas to ever catch. Any fast, docile small fish could work. Avoid larger fish, fish with long fins, other anabantoids, etc.

I used to have an undergravel filter and didn't like it but some people love them.