My 2 cents
I have a 1 ac pond. I've stocked 14 GC 15" in 2013 and 7 more last year.
In 2017 I found one of the initial stocked floating and it was 34"x24 lb. I've seen others swimming around in august and they were about 40" long.
My pond shore is covered in phragmites for the 95% and the 20 GC are able only to keep it where it is, eating waterside new sprouts, some leaves and rarely some whole plant.
A friend of mine had instead a 2 ac pond with little vegetation problem on the shore. He put in 8 GC 15" long in 2014. After eating every single plant on the shore in the first year they start to heavily dig the shore looking for roots. Now his shore is collapsing in several spot, mined at water level, and he's trying to remove them but it's really hard.
So think about adding GC carefully.

Filippo