There are alot of very compatible fish you can keep with a betta. In reality alot of fish you need to watch out for may be fish that would bully your betta. Male bettas are mostly just agressive to other male betta splendins, but I would be careful with Gouramis or any other labrinyth fish. If you keep female bettas at all sometimes they can be a great addition to a community tank. A 25 gallon should give you ample room for some cool stocking options. You should also plan on a cleaning crew; ghost, amano, or cherry shrimp, a cory catfish, and a plecostomus of some kind, possibly a clown pleco. (Most species of ancistrus stay between 3 and 4 inches which would be good for your tank size.) The coolest bettas in the world are the wild type species. Betta coccina, smaragdina, imbellis and the full line of mouth brooders can usually all be kept in groups. For the most part it is just splendins males that will fight to the death. I have some wilds, pm me if your interested.