Betta's

They can do well with alot of community fish as long as their is ample space. You would want to stay away from any gouramis of fish with flowing fins. I keep my male betta with cherry barbs, cardinal and neon tetras, and cory catfish with no issues whatsoever.
 
I've kept about 10-20 bettas over the last 5 years and I've kept them with almost anything from dwarf gouramis to barbs. Really the only thing that ever nipped them was cherry barbs. Everything else was really fine.
 
They have highly variable personalities, it really depends on the betta. In general though you should avoid tankmates that have large, flowing fins, are very colorful (betta colors), and are prone to nip fins.

For example I've got one betta who is a great tank mate with a Paradise blue gourami, however another betta who sees this same gourami as something that absolutely must die. I'd recommend you have a backup plan (return policy on purchased fish, or another tank for them to live in) if you're going to try to mix them.
 
every bettas is different; some coexsist fine with other tankmates, but some are so aggressive they flare at heaters.
That made me laugh. Ok, I'll see how it goes. The reason I ask is I bought a breeding pair from AB and they are in their own tank but he sent an extra male (I think because he wanted to make up for how long he took to ship) which was very nice of him and I had nowhere to put him except for my 40 community. He seems to be doing really well and nobody is having any problems yet.
 
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