If it is a male then you have to keep away from any type of fish known as fin nippers, such as Barbs. It does not always happen, but it is a gamble you can do without. If you want to add live bearers, such as Platy, Guppy or Swordtail, it should not be much of a problem unless you want to keep your fry, the Betta will find them wherever they hide, believe me, I just lost my last fry, which had hidden themselves perfectly, but along came a Betta...
Depending on the tank size, smaller schooling fish such as neons and Tetras!
Best bet is decide on the fish you would like and then ask here if anyone has any experience and I am sure someone can help you!