Mmm. I've read that bettas usually compatible with other fish as long as the other fish don't look too much like bettas and won't nip the betta's fins.
It depends on the individual betta's personality. Some do great with tankmates, some will kill anything you try to add to your tank. I was looking at a betta for my 5 gallon that had some snails but decided against it because I didn't want to risk getting an aggressive one and having him beat up my snails.