Firstly, as some others have mentioned, don't expect any creature to significantly clean up the tank, regular tank maintenance is your responsibility. And another common myth is there are certain fish that eat poop, there's no such fish.
However, a clean up crew in a tank does help to some...
At least 5 gallons for each puffer. If keeping more than 1 puffer and not enough space, then a puffer could very well decide to take over another puffer's 5 gallons. Civil (or very uncivil) war rages!
Bettas can be real fussy about food. Personally I would recommend a staple diet of Hikari Bio-Gold betta pellets, just a few pellets once a day or every 2 days. And occasional treats of frozen or dried bloodworms. Live mosquito larvae would be a food they're used to in the wild, but suggest not...