green and pumpkins will grow about the same size, but pumpkins potentially get over 10" which is doable but will start to be tight. Greens will be fine for life, but if a pumpkin grows to its full potential it will be cramped in a 55, but that will take years. In my opinion and experience, your tank will look most natural and most interesting if you got with a community of shiners, dace or minnows. If you add a sunfish, small species will get eaten. IMO, shiners are messy and sporadic. All baitfish for that matter tend to be sporadic, but dace and minnows tend to settle down a bit more than shiner species. You also have a native killi which make great aquarium species, and also a native madtom which is a very small catfish perfect for a 55
No madtoms in my area that aren't endangered, other than stonecats which get a bit big. I do like the shiners and dace idea. You think a green would be fine in there?
Here is what I could do (hear me out). Go get all the stuff for the tank and set it up. Get the plants and plant them. Cycle it. Get a single green sunfish and grow it out until I get my 180 which will be either natives or cichlids (I've heard many cases that they do well together). Only thing I worry about with that is the solitary life for the first year to two years would make him/her to nasty to be moved into a tank with other fish,