You could fit one oto cat in the 5 gal
betta tank. The betta will ignore him. Just be aware that you will have to start feeding the oto sinking algae wafers when the diatoms are all gone.
I agree with the comment about your changing filters so often. So long as there is water flow going through it, the cartridge is doing its job. Instead of changing cartridges, I just rinse mine out in tank water every so often.