Threadfins and gizzard both are the worst thing you could ever do to a pond if you care about big bluegill at all. They're great for growing big bass, but to answer your question, GTS, they most definitely eat plankton, like it was going out of style. I'm working with a 56-acre lake right now that has small bluegill (average 5" or so); one of the first things the owner told me the first time we talked was that his lake used to have "huge" bluegill but that he didn't know what had happened to them but they were a fraction of the size they used to be. He had mentioned that he had stocked threadfin and gizzard four or five years ago, so I asked him if that was about the time his bluegill started declining, and he said, "Oh yeah, come to think of it, it was." He likes catching big bass but also is an avid bluegill angler; the people who managed the lake previously had just not asked enough questions, or else had not listened too closely to his answers.
As to GSH, they too most definitely compete with bluegill for food. There's a lake around 150 acres near my hometown that years ago, before TWRA took it over, produced bluegill approaching three pounds - I've seen two of them mounted, just beasts. So TWRA stocked GSH in the lake, and now it's overrun with them, and the bluegill are a fraction of the size they once were. I fished it once this spring, fished several different spots all around the lake, and I couldn't even get my bait to the bluegill because it was being taken almost every cast, everywhere I fished, by 8" GSH. The few bluegill I caught were small, under 6". I haven't been back since. There are a few big LMB in the lake now but the GSH have ruined the bluegill fishing.
I've never seen a lake where threadfin, gizzard, or GSH were stocked that the bluegill average size didn't plummet.
