Bluegills make good cutbait at times for channels and blues. If they are small enough I'll cut the head off and hook it through the eye socket and use it for bait and then cut the rest in vertical chunks. I also cut off the fins and cut most of the belly portion off where the intestines are because there isn't much there and just balls up in a fishes mouth. Nothing sucks more than waiting a long time and when you finally get bit you set the hook back into the bait. Run the hook through the top portion of the bait, don't try to hide the hook, make sure there are no scales on the tip of the hook. You can scale them but it's just extra work you don't have to do. Most of the bluegill I use for cutbait are 4-7 inches long, smaller ones are too soft for me. If I can get away with it I'll fish cut bluegill with no weight. It casts fine without it and sometimes I'll even fish it like a plastic worm, letting it sink and then very slowly jigging it in.
For the flatheads use live bait. I prefer sunfish over bluegill, they stay alive on the hook longer and swim harder. Bullheads, carp, drum, shad, basically anything they can fit in their mouth that is legal bait works. If there is no current I hook them just behind the dorsal fin towards the tail, about half an inch from the back. If there is current I hook them through the nostrils. Set them up on basically a carolina rig with a short leader, 10 to 12 inches is plenty in most cases. If there is lots of snags then use the shortest leader you can tie, flatheads aren't scared of the sinker. For the baits I use it takes about a 4-8oz sinker to hold them down. Make sure to use heavy gear. Flatheads are bullies that live in tough neighborhoods. They aren't going to fight you for 20 minutes like a big blue will but for the 5-10 minutes it might take they are going to make you earn it. I wouldn't use anything lighter than 30lb test, my flathead rigs have between 50 and 130 pound braid on them.