C. fowleri is one of the larger Cories. I would not want to see it restricted to a 10 or even 15 gallon tank.
If a 15 is the largest you can use, then perhaps you would look into some of the Dwarf (several species) or at least the smaller sized Cories. Then you could get enough for a decent school, and see some really nice activity levels without stressing the bioload.
Also, many Cories tend to prefer tanks that are cooler, many no warmer than the mid 70sF. For Bettas this would be the low end; though they can survive cooler water, they are not very active.
I would suggest looking for Cories that thrive in the upper 70s to be more compatible with the Bettas.