I use my index and middle fingers with my thumb for both tomahawks and thumbers. As with forehand throws, I keep my fingers stacked against the rim for tomahawks. The flight paths of the two throws could be considered mirror images, so it's definitely worth learning to throw both.
Overstable discs will turn more slowly when thrown overhand, while understable discs will turn and flip more quickly. The basic flight pattern will be for the disc to turn upside down in a broad, descending right-to-left (RH thumber) or left-to-right (RH tomahawk) loop, and if the flight plate continues turning past horizontal, it will finish by dumping back the other direction and tending to hit the ground edge-on. For right-handers, tomahawks tend to finish left of the direction you throw them, and thumbers tend to finish to the right of the direction you throw them, unless you throw it so that it hits the ground before it sweeps past upside down.
All this can be manipulated by the release angle. You could even think of the transition from forehand to tomahawk as taking the release angle past extreme anhyzer. You can throw it with the edge to the sky, or throw it over your head with the flight plate facing down, which can work as a sort of overhand "scooby" throw. It's hard to get an overhand throw down a tight tunnel, but if you throw an overstable disc hard and low, you can get some interesting results. Be ready for the disc to skip if it hits the ground upside down, or bounce and roll a little if it hits the ground on edge. Have fun experimenting.