If you put a bullhead in that tank with darters, you might as well kiss them goodbye
Many would say the same with Angelfish and Neons or Cardinals, but that is done with much success on a regular basis. Bullhead are opportunistic, not predatory. Altho they will eat anything that fits in their mouth, I don't believe they would go chasing after something as fast as a darter, when there's other easier to catch foods readily available. Surveys done on the stomach contents of bullhead didn't come up any higher than 6% fish. I've got mixed reviews on this one tho. I'm doing a bit more research.
Pumpkin seeds are territorial.... You will probably end up with a pair and three not so happy fish.
Only when breeding. Otherwise juveniles hang out in shallows in relatively large groups, and adults hang out in deeper water often in pairs or even small groups up to 6 or more. As for if I end up with a breeding pair, I'll deal with it when the time comes.
That filtration would not cut it for what you are proposing.
The tank will be started with relatively low bioload, an established biological substrate, and dense planting. I'm sure the filters will do fine, and if they cant quite keep up as the fish grow, I've got others on standby.
Darters need high oxygen with cool pristine waters.
You can't just lump all darters into the same category. From everything I've read the Iowa darter prefers clear STANDING or SLOW MOVING water with dense vegetation, and sandy bottoms, and enjoy temperatures ranging from 1 - 36 Celsius (33.8 - 89.6 F). Not only that but coming from northern climate where their habitat freezes over in the winter they do fine with low oxygen levels.
I wouldn't be starting off with all adult fish here, with the exception of the darters. Both the sunfish and the bullhead would be juveniles. The sunfish would be 2-6in, and the bullhead would probably be around 4-6in. The adult darters would be around 2-3in. I'm researching these fish and their habitat, not just randomly throwing them together.
Final note: kwheeler91, please don't be offended by my debating. I appreciate the input and respect your opinion. I am only in the very beginning stages of planning this tank and have LOTS of hours of research ahead of me before any fish go into this tank. Thanks.