they sure make good bait on the missouri river! I don't go catfishing without live green sun's. we call em "black perch" around here. i like using em from about 5"- as big as I can find. I'd say at least 2 out of 3 flathead I catch is on a live geen sunfish. I catch lots of bigger channels and blues on em, too, but I am targeting flatheads when I use em.
as for rigging, try using one under a big float, about 3 ft deep with a heavy splitshot halfway to the hook. hook him in the back, above the backbone. fish close to some shallow wood cover. let him swim around some. dont forget to back your drag off some and put your rod in a good rod holder.
if I am fishing on bottom prefer a slip sinker rig, with the bait hooked through the nose, or eyes. you have to be very gentle hooking them this way, dont tear any flesh, or the bait will get off. a 3 way rig also works fine.
with live bait, you need extra patience, because the catfish are actually "hunting" your bait. and dont set the hook until the kittie cat is swimming away. the wait will be worth it, I swear!