You obviously have not thought this whole thing thru!
2.5 gal pico with a few gallons worth of a sump, and the two 1/2 gallon betta bowls, you could ditch the sump, ditch the 2.5 pico, ditch the bettas and use those fiters for now and get a tank equivilant to the amount of gallons as stated above. So you should be a ble to get a at least a ten gallon or so.