Definitely don't mix a male with females. You're quite likely to end up with some dead fish if you do that. If you're careful, sometimes you can house female bettas together, but not with a male.
If you're thinking about upgrading to a bigger tank and getting more fish, I suggest getting the biggest tank you can afford/have room for. Bigger tanks are generally easier to maintain because they're more stable than small tanks. They also allow you to keep more fish!
For example, if you got a 29 gallon tank, you could keep some interesting stuff:
-A big school (8-12) of corydoras catfish
-Another big school (a dozen maybe) of peaceful schooling fish like harlequin rasboras
That would make for a really interesting tank. If that's bigger than you've got room for, you could still keep a betta in a ten gallon tank with a small herd of pygmy corydoras.