A drywall pro hanging drywall to uneven studs will use drywall shims to get a planar surface. I would think you could do that in your case.
Using 5/8" drywall would almost eliminate the need for shims because it is so stiff.
But why are you putting drywall over plaster? I'm not questioning your good sense, but your reason may be important in other ways. This creates a problem at windows and doors, to say nothing of electrical boxes.
On my website at www.bobf.info I give a lot of directions on working with drywall, and texture, and painting.
It is very hard to follow a thread on this board, so if you have other questions, contact me through my website.