corys are great little creatures and should get along with your betta. Pygmies are a bit delicate, so I would go with either sand or small rounded gravel. Whatever fish you decide on, make sure to add the non bettas first, so the Betta doesn't try to defend his territory. I lost a cory once because of that mistake :(
Ares - My flare happy warrior
Castiel - My inquisitive crowntail
Sammie,Jackie, and Jamie - My fun loving peppered cories.
Touchstone - My sweet fool
Freyja - My beautiful girl