Steve,
Others more knowledgeable will have to fill you in on the stocking rates of the larger bluegills. However it can certainly work as I started a pond this way a later planted hatchery supplied bass. Typically it is better to get a particular size and number of fish from a hatchery but as you say it is hard to find regular bluegills in certain parts of the country.
I too ran in the pushing of those damn hybrids over regular bluegills. They have there place in small ponds where the pond owner is interested in only bluegills, but I think there is a lot of salesmanship and misinformation out them telling folks that hybrids are superior. If you want big bass you can forget about using hybrids. They won't provide enough forage for the bass.I wouldn't have them in my ponds.
I would avoid pumpkinseeds too. They do not grow as large as bluegills and tend to hybridize with your bluegills.
If you have a gravel pit have you considered smallmouth bass and yellow perch? Smallmouth can be hard to find too but I do know of a few sources.
