Hi bluegillkiller. Bluegill are my specialty. All the advice you've already received is right-on. It's important to keep in mind that both big bass and big bluegill in one pond are very difficult to achieve; it can be done, but it's not common, and probably not a realistic goal.
From what you've described, I would tend to agree with Sunil that your bass are overcrowded; all the symptoms point to this: big bluegill, thin bass mostly smaller in size. Something very important to understand: if bluegill are top priority, don't remove any bass from the pond. One of the first steps in managing any pond for trophy bluegill is to let the bass overcrowd, because when they're overcrowded they keep the bluegill well-thinned such that the bluegill that survive the LMB gauntlet have all they want to eat and grow very large, several times what they will in a pond or lake in which the bluegill are overpopulated. So if bluegill are the main species of focus, don't keep any bass. Not one.
You're right to release the bigger bluegill; they can't get to 10" if they're yanked out of the lake when they're 8". Culling can also help; I discovered this by accident on the best bluegill lake I ever managed. It was four acres, and the first two years I worked with it I kept close to the recommended poundage of bluegill per acre for a fertilized pond in TN, according to TWRA, before I knew that was far too many big bluegill to keep per acre; I fished down the pond to where you had to work to catch fifteen or twenty bluegill in half a day, but those fifteen or twenty averaged fourteen ounces or better every time for several years, and I caught at least one pounder every trip, and many far over a pound.
Other things you can do to increase the average size of the bluegill include stocking grass shrimp (P. kadiakensis) as an extra forage source for them, and feeding high-protein pellet food as Sunil mentioned, preferably via an automatic feeder which can be programmed to feed multiple times a day and which thus is more consistent than any human. The pellet food will make a bigger difference than anything else you do other than not keeping bass; if you do nothing else new other than install three to six or eight automatic feeders and begin feeding two or three times a day (less or none in the cold months, depending on the fish response), you could double the average size of the bluegill in two years. Make sure to get a high-quality food, as all foods are not created equal and this makes a big difference; most of us on here use Purina Aquamax 500 for bluegill, though you could mix it with 600 since you have some bigger bluegill already.
