I keep a comet with my tropical community, they can live in either warm or cold water and do well. The only real issue with it is the ammonia, that's it IMO. But if you have that under control then you're fine.
And as for the betta, normally it's a no-no, but I've had it work before. I've also had it fail terribly before.
The whole thing with compatibility is the fish itself and its own unique personality. When they say "those two fish won't work together", that normally means that it won't work most of the time (unless we're talking about more aggressive fish. I'm only refering to most community fish).
And when dealing with community fish I always say, there's an exception to every rule, but the question is is it worth the risk?
In this case I would say you're fine, but be ready to remove it quickly if things don't work out.
Just keep in mind that everything I said isn't on the books, it's just my personal expiriences and opinions. So don't go to the bank on anything
