FBT should be kept alone. I've read about keeping small fast fish with them in a tank with a large water section; but it's a risk to the fish (FBT toxins in water). Also, you have to consider the risk that fish might carry parasites that could affect frogs.
For inside aquarium filters, prefer the Supreme Ovation. Recomend over-filtering frog's water around 10X the filter rating; so the Ovation 1000 can easily handle 8-10 actual gallons. If water feature area will be smaller than that; there are smaller models like the Ovation 700 which is good for around 5-6 total gallons. To estimate the actual volume of water, need to measure the area you will use for your "pond" and enter into this site for calculation. Good luck!