Harvest some of the bass IF you want a trophy bass pond, or if you want to catch some good bass and possibly some decent bluegill. However, if you want trophy bluegill, harvesting any significant number of bass will make the situation worse rather than better as the bluegill will overpopulate, and probably pretty quickly. Rule number one of growing large bluegill is to ALLOW the predators, in this case and usually in ponds LMB, to overpopulate, so that the bluegill cannot overpopulate; the bluegill that live have all the food that they can eat, and grow very quickly and will reach sizes several times that of a pond with any other population dynamic, including a balanced one. This is widely accepted in the field of pond management, and is espoused by everyone from the founder of this site to just about any state game and fish agency that offers specific advice on growing trophy bluegill. Here again, if trophy bass are your goal, by all means you need to harvest smaller bass and several of them, because you want fewer bass; the bass that are left in the pond will have all the small bluegill their hearts could ever desire, with none of the bluegill exceeding the size they can swallow.
If, on the other hand, bluegill are your first priority, you may need to apply alum. If by murky you mean tending towards muddy, muddy water doesn't have much plankton, which is the base of the food chain in a pond, and a primary food source for bluegill. If your water is muddy, your bluegill could be small simply because that's all they've grown in two years due to not much to eat. If you get your water cleared up, AND maintain the bass-heavy status so the bluegill don't explode in population like Ewest cautions against which could easily happen if the bass are thinned, then clearing the water and then fertilizing will give you a food chain about ten times more productive for the bluegill than what you have right now. Assuming you truly are bass-crowded, which is key to this equation, the bluegill will start growing much, much faster. If you then feed on top of this, they'll double or triple in size within a year.
They will grow much faster if you feed, even without fertilization. But it sounds to me like right now they're small simply because you have a bad food chain for them, i.e. not much plankton.
Can you take some photos of your water?
