i reckon it would get a bit "dicey pricey" sorry for that rubbish joke. But if i remeber Congos have long finnage and the betta would love to have a free meal. or the other way round:s
I know it's been done, but I would advise against housing a betta with fish that have long, flowing fins. I keep my betta with some cories and he likes that just fine, but when I tried to introduce a couple swordtails, he wanted none of that. Bettas really aren't the best community fish.