I was going to make a new thread and ask a very similar question. We have a 50g tank and have recently finished the cycle. Right now, there are 7 neon tetras, 2 dwarf gouramis (1 neon blue, 1 flame both male) and an angel. All fish are relatively small, less than 2 inches. We let everyone get accustomed to the tank and decided to add a half moon male betta. After about an hour or two I noticed the betta was acting aggressive towards the flame dwarf. It wasn't all the time but it only took twice for his gills and tail at full extension for me to yank him out of the tank and place him in a small betta bowl. The gouramis were not picking on him and they do fine with the angel. I have plenty of hiding places suitable for everyone. The other thing i thought was strange was that the betta didnt seem to mind the neon blue gourami, just the flame. I figured in a 50g it wouldnt cause any problems. I'm thinking of waiting another day or two and trying to add him again... but i'm also thinking of returning the betta to the LFS. I really dont want to do that but I dont think i can keep a betta bowl around either. My cat has shown great interest in the betta since we've placed him in the bowl.
In the past though I have had a betta in a 10g with a bunch of guppies and various tetra. I was about 14 or so and didnt know about guppy reproduction. Needless to say, when the female guppies were spawning, the betta was not far behind enjoying a snack. They survived together fine except for the fry. Good luck! I hope you have him some good tankmates.
Haha! That's what i am scared of!!!! Making a plan for fish around him and then him not working... i wouldn't even know what to add to replace him. If that does happen i will probably get rid of the betta... i can't stand having just ONE fish in a tank... very boring