Hi,
if you have enough room, go for the 10 gal. In a bigger water volume, it´s easier to maintain a good water quality. I also would suggest to keep more than one ADF (ACFs really get large and they like to swim- altough 10 gal are often told to be sufficient for one ACF, I wouldn´t like to cramp them into a tank this small. 5 gal are an absolute no-go for those frogs in my opinion!) in a species-only tank as they enjoy to be in groups and might be shy if fish are present. Also, they are slow feeders, so most people with frog-fish-community tanks end up with either skinny frogs or fat fish. Mostly both

ADFs are best fed with a variety of live and wet-frozen foods; avoid to feed too much bloodorms, as they seem to promote bloating in pipid frogs. There are many other possibilities: glassworms, black mosquito larvae, daphnia, cyclops, tubifex (look for a good quality), mysis, brine shrimp and many more. Small pieces of earthworms are rich in calcium, which can prevent health problems. Some ADFs don´t feed on pellets; if you want to use those, buy small ones that are made for amphibians. Still, I would recommend to feed other foods as mentioned above as a basis and only alternate with pellets from time to time.