It really REALLY depends. Female guppies are safer to put in than males, but it really depends on your boy. (and be sure to have LOTS of hiding places)
Every betta is different, one may do something that goes against the rest, but I wouldn't put my luck on it. Why don't you try something like a bright colored Platy first to see if your boy tolerates tank mates?
I would take him out, rearrange everything, then stick the fishies in first. THEN add him. If he stalks and nips them for more than a few hours or is seriously aggressive then I wouldn't add anything.
My boy seemed really docile until I tried moving him into my heavily decorated 10gal platy tank. He ripped one of my boy platys apart....*R.I.P, Pong.*