Good questions, DU. There have been studies done on the negative impact of shad on bluegill growth. Here are two on the negative effects of gizzard shad, both in ponds and reservoirs; one study was in NE, the other in IL:
http://cat.inist.fr/?aModele=afficheN&cpsidt=14662602http://afsjournals.org/doi/abs/10.1577/M04-008.1Threadfin shad and GSH are not mentioned, of course, but they eat the same foods as gizzard shad; the key is the competition with bluegill for food sources. Every time I've ever fished a lake in which any of the three had been stocked, the average bluegill size was cut at least in half of what it had been.
Tilapia are perhaps a slightly better option, as far as the bluegill are concerned, but they love pellet food and will thus compete with the bluegill on that front.
It is very difficult to manage for both trophy bass and trophy bluegill. The two ways to go about it are 1) regimented harvest of both such that neither ever becomes overcrowded, a "balanced" pond such as most state game and fish agencies used to recommend, and 2) stocking of a predator above the bass, northern pike or tiger muskie being my preference, that will keep the bluegill and bass both thinned. With either method, you probably wouldn't get LMB as big on the top end as you would with a very forage-heavy approach, but you'll still get bass much bigger than if you let them overcrowd to make the bluegill bigger, and you'll get huge bluegill if you feed and fertilize.
I would say, if trophy LMB are your primary goal, go ahead and stock tilapia, and probably also GSH; with regular feeding by automatic feeders, you can still get some big bluegill, they just won't be two-pounders. If you want two-pound bluegill, though, don't stock anything that's going to compete with them for food.
