A lot of this boils down to personal perferences and what your objectives are for the pond.
Some people want to raise fish for food ...others want to have sport fishing...and there are an infinite number of combinations between the two views. There are also infinite permutations, i.e. different types of food fish and different types of sport fish that make a "one size fits all" an impossible situation.
If you know what your objectives are, there are any number of documents available to provide numbers of fish per acre. An internet search will yield many such documents. One I like is this one:
http://www.sdafs.org/tcafs/manuals/tcafs_pond_manual_2005.pdfAnother even better one is a CD put together by Texas A&M on "Better fishing through private water management". It is excellent, but may not be currently available to the public.
Regarding green sunfish, again this is largely a personal preference. Some people regard them as "trash fish".
I am
not one of those. I happen to highly respect their aggressive nature, their willingness to take artificials and flies, and their fight which is superior to most all of the sunfishes.
Fish simply do what their genetics dictate. Put green sunfish in a pond without adequate predators and they will overpopulate and stunt. Put coppernosed bluegills in a pond without predators and they will overpopulate and stunt.
Coppernose bluegills reproduce much faster than green sunfish and hence are preferred for ponds in which forage generation is important....again depends on your objectives.
Decide on what you want from the pond...and there are several good people on here that can offer you advice on what has and has not worked for them. The decision is relatively important because once stocked, it can be expensive and very time consuming to change directions....been there and done that. Good luck.