Whisper makes and in-tank type HOB filter that works in either a full aquarium or down to 2 inches of water... they make them 5-10g size and bigger. One should do your tank nicely.
I'd think you could put maybe 6-8 dwarf frogs in a 10g, depending what else was in there it might be less though. Maybe more, they aren't that messy, aren't that active, and sure aren't territorial.