You really should focus on making the school of cories as large as possible rather than adding another species. Rasboras are a schooling species also but should not be kept in a school smaller than 8.

With schools, your number 1 priority is to make them as large as possible. I still feel that if properly filtered your system will be just fine with 6 cories. They love groups and there's no such thing as territories or too many buddies for cories.

If you up your filtration or add more plants (and are diligent with water changes and monitoring your parameters)....well if this was *my* tank, I wouldn't hesitate to do both schools. I am comfortable with slightly overstocking my tanks as I am manic about water changes, testing my water, and all of my tanks have live plants in them.

Again, that is your choice and only you can make it. I do recommend focusing on the schooling aspect rather than the diversity aspect.

As far as your frog eating...YAY!!!!