dollar sunfish are pretty small and only reach around 4" max. pumpkinseeds CAN get big but they are generally pretty small under 10 inches.
all sunfish are aggressive and i think dollar sunfish are some of the more aggressive, but i'm not sure. this could easily be found out though. pumpkinseeds aren't too aggressive but they can be.
since i don't know how aggressive dollars are, i don't know how many you could keep in a 55, but you could keep about 3 or 4 pumpkinseeds in a 55.
for sunfish, they need space, lots of food, lots of cover, and good filtration. they'll eat any kind of bug you throw in the tank, any kind of aquatic insect you can find, earth worms, small minnows, nightcrawlers, crickets, mealworms, etc. just about any kind of live food. frozen bloodworms are a favorite too. they will also eat flakes but these don't really do much for them, and its in your best interest to train your fish to eat floating cichlid pellets while they are still small, because it will be easier at this time, and more economical down the road. i have mine eating pellets as a staple, live food whenever i find any, and a weekly treat like frozen shrimp (thawed).
room temperature water is ideal for sunfish, no heater needed.