I'd recommend getting a 10 gallon at the least. Worst comes to worse Wal-mart sells a 10 gallon kit for 27.00 and I am sure Petco/Petsmart has some equivalent. it comes with a strong filter which is nice and hood. I'd recommend buying some live plants and get the hood that has flourescent lightbulbs so you can stick some 6500 small lights in it. The live plants will make your fish happy and eat some of the waste from your fish to grow with. If the filter is to strong for your betta you can baffle it with a aquarium sponge and a rubber band, pretty gheto solution but it works great and adds another media to your sparkling water.
Otos are tiny but they produce a lot of waste because they never stop eating, I had 3 in my 10 gallon tank with white substrate and hated it, all i could see was poop everywhere and had to clean it daily. I moved them to my bigger tank with the other school of otos and they are all fat and happy in there but the algae in my 10 gallon is also happy and driving me nuts so I guess the otos are going to go back in the 10 gallon - they do an awesome job of keeping it sparkling clean. The difference in 1 oto vs 3 to 5 otos is amazing. They swim and run across the tank like crazy together where as 1 will normally hide and not move out of a couple spots very much. They just look like tiny balls of HAPPY when in a group together - 3 is enough for them to be happy 4 or 5 is even better. In a 10 gallon I'd go with 3 at first, you can see how they do - that way you don't have 5 fish you can't support in there.