I'm sure an ADF will cannibalize if the other frog is small enough as most amphibians tend to do.. I believe the general rule of thumb is that most animals will take prey that is no larger than 1/3 their size (then again some amphibians have huge mouths.. but ADF don't).. I would make sure they're roughly the same size to play it safe.. no less than 1/2 the size of your current frog should work..