Fish I have had with my bettas and done fine with and some are really remarkable to watch:
Any type of Corydoras catfish in the size tank you have. The thing is with cories that they need to be in groups of at least 3 and like it better in larger groups. If you choose the Pygmy cories they need at least 6 and like it better with 9 or more. They are very entertaining but will not eat just scavenger type food you need to actually supplement them with protein as they eat shrimp pellets, bloodworms and other carnivorous type foods, they are not algae eaters. There are a LOT of kinds of Corydoras and a book I would recommend is
Identifying Corydoradinae Catfish by Ian A.M. Fuller & Hans-Georg Evers. It shows the diversity in color ranges and patterns of this lovely fish. We have members who breed the cories so they would probably be easy to find. They are not usually expensive fish but oh so worth the effort. Most can be found on Catfish websites on the internet if you need more information about them. One of the better sites for catfish information is:
PlanetCatfish • the online home of aquarium catfishes
Otocinclus catfish, a bit delicate and nocturnal creatures but never met a betta that did not love their otos. I have pictures of one of my bettas taking an afternoon nap under the heater with his otos. Two little butts under the heater one betta one oto. Darling pic. But if you want entertainment these little guys do not come out during the day much. They can keep brown algae down but do not do much with the green types. They have had rough handling and are wild caught. If you want to hear their sad plight they do have their own website at:
www.otocinclus.com - Home of the Dwarf Suckermouth Catfishes I personally love them they are sweet little fish but be cautious when you buy as there are warning signs to get a healthy one. No bright red on belly, no sunken belly should have a rounded belly but not bloated. Eyes should be bright and color should show a definite stripe down the side not be muddy in color.
Dwarf or Chain Loaches - very active and beautifully marked fish. They stay very small in the 2" range and believe me the more of these guys the more activity you will see. A group of 5 or 6 will keep you rolling in your seats. They can go for hours and you wonder where they get their energy. They do not seem to bother the betta as they climb the wall.
My betta would just hang there looking on disdainfully but not bother them and more importantly they never bothered the betta.
Kuhli Loaches - basically the same thing but not so much activity and more nocturnal but kind of fun. Not so nicely marked and not as much activity but some.
Cardinal Tetras - These are the only fish of the tetra variety that I would put in with bettas as they are the only ones that I know of that are NOT fin nippers and would not cause your betta fin problems.
Now for the fish that either I or others that I am aware of have had problems with:
Platies resemble female bettas and for that reason are not good choices. Potential for either fish to have problems.
Mollies are brackish fish and also have long flowing fins and are sometimes mistaken by Mr/Ms Betta as another betta.
Guppies are not good candidates as they have been known to be nippers and can also have long finnage and confusion about being another betta ensues.
Let me just say here to save a lot of time ANY fish with long or flowing fins is not a good candidate for a tank with a betta, no other betta male or female is good, no related fish like a gourami is good, no other potentially semi aggressive fish like sharks, angels, cichlids, puffers, barbs, crabs, lobsters, or fish of obvious diversity in size like discus.
These fish while they may get along in a community setting have not been raised in one and as such need peaceful and non obtrusive fish in their environment. The fish that I have listed are some very active but they generally do not encroach upon the territory of the betta, but it would be my suggestion if you are going to put the betta in with other fish that the betta be the last fish you introduce into the tank. You will have a better chance of him being peaceful with the others if they have established territories and he is entering into a place where he is taking what is left rather than him believing the whole tank is his and seeing THEM as intruders in his territory. There is absolutely no doubt that they betta will fit into what ever he is given. But if you start by giving him the whole tank that is what he is going to see as his. If you give him the part the others have not occupied he will be happy with that too.
Hope this has helped.
Rose