Small, fast moving fish that have small fins and are non-aggressive.
This way: even if the betta hates any and all other fish it won't get beat up when it chases the others, it won't be able to noticeably tear up the fins of your other fish, and if you have many of them it won't be able to harass a single fish to death.
Probably could toss in a variety of neon, cardinal, green fire, glo-light, or black neon tetras. I don't mean throw everything in there, but it is a list of smaller tetras who prefer groups of like fish that would work well as I described previously. Maybe pick 2 species and build up a group of 6-10 of each...of course this is with the idea that these would be the only inhabitants of the 29G.
Platies and swordtails may be a bit slow moving and easy targets for an aggressive betta...if your betta plays well with others than it could be no problem but I personally wouldn't bother with the risk (fish just look terrible when their fins are getting torn to shreds).