Bettas can make great additions to the community tank, but their tank mates need to be chosen with care and the bettas personality taken into consideration.
First, is your betta highly aggressive, or has it been alone for most of its life? Some bettas are solely solitary fish. Adult males can be aggressive to any fish, whether it is the same species or not. Males that have often been alone for several months may not welcome other fish into “his” territory. If it is an old fish who has been alone all of his life, then it is better to leave him that way. Old fish may become stressed when surrounded by new, curious fish, whose motives he isn’t sure of.
When picking out tankmates, be aware of all of the fish’s personalities. Watch for fish that might hurt the betta. Fin nipping fish such as barbs and most tetras will often rip at the males finnage, which can lead to the male killing its tankmates, or the male being killed from damage and infection from his torn fins. Gouramis are never good tankmates for bettas, as they are anabantoids like the betta. Gouramis will fight, and often kill, bettas.
Other aggressive fish (cichlids, especially when spawning) shouldn’t be part of the bettas tank, as they may get into a territory battle. Next, consider whether or not the betta will be a danger to the fish. Fancy guppies and lyretail swordtails, with long finnage, may pose a challenge to the betta and be hurt or killed. Small, darting fish (mosquito fish or small neon tetras) may be mistaken for food and eaten. Livebearer (molies, platies, guppies, swordtails) fry will be swallowed eagerly.
So what kind of fish can you have with a betta? A whole bunch of kinds, actually. Catfish are fine, and are cute as can be, too. Livebearers (besides those with fancy finnage) make great companions, as long as you don’t expect the fry to survive. Danios and rainbow fish are peaceful and quite beautiful fish. Larger, non-aggressive tetras and barbs (adult Neon Tetras, Cherry barbs) make fine tankmates, as do plecostomus and many sharks, such as the bala or red tail shark (many sharks get quite large though, so be sure that you have enough tank space for the adult sharks). Shrimps can be good tankmates, but some bettas will eat the shrimp.