Ignore those that say a betta will instantly kill any other fish they encounter. They can make good tank mates, if a few things are done. No other fish with long, flashy fins like fancy type guppies. Also no barbs or mollies. Both are known fin nippers and the betta will be the one in trouble. Some people say not with gouramis and this might be so. i never tried it. But usually with tetras, rasboras and platies, they seem to get a long. Now, that said, it all depends on the character of all fish included. If its a especially aggressive betta, then he might have too many hormones rolling around and would be better off by himself to begin with. Also other types of fish can be aggressive when they are known to be so. Nothing is written ins tone, but there are indicators and that's all we can give you. Make sure the tank is not crowded, there are plenty of plants and other things for hding places and things should work out.