Mine can hit it pretty hard. The 12" deep rule comes from every frog forum I've found (not Yahoo or general forums). When I first got the ADFs I thought they were dead when they were in what I call their "Zen" pose and floating in weird positions. Their swimming abilities are amazing.
My community tank is right next to my end of the sofa so I can do a lot of observing of different interactions. The ADF don't bother any of the other fish or the shrimp and the Betta doesn't bother them.
How big is your tank? Some frog forums say they can be in less than a five but with the way they swim and putz around, I wouldn't put them in less than a five.
They are shy and do need hides and plants. You may also have to target feed them with long planting or reptile tongs or the Betta will steal their food. They do fine on frozen bloodworms.
Read this for information from those who keep them and FAQs:
http://www.bettafish.com/showthread.php?t=84033&highlight=african+dwarf+frogs
BTW, this particular person says 18"-24" depth is fine.
