RS I ve put them (20 GC in a 1 acre pond) only because I have almost all the banks covered of Phragmites palustris, sprouting everywhere.
GC, despite being so many, can barely keep it under control.
A friend of mine put them in his pond (very steep banks and very little vegetation) and now the banks are collapsing due to the CG digging in the mud looking for food. But he put a lot of GC too.
Maybe only 2 won't cause any disaster but...keep an eye on them.