6" drain pipe? And that's the overflow? I basically no nothing about ponds but what I see around Elkhart. Usually a 12" of 16" drain, some sort of siphon set up. Never seen it work but the drain goes near the bottom, people say it'ss drain a pond in no time. As far as overflows, the more the better to keep the water off the dam. One friends pond used 4 24" pipes for overflow, water diverted down a concrete shute. This guy has built many ponds and says to much is not a bad thing, also it depend on what the water shed is capable of producing in a hard rain. This same lake has an second lake that's further up the hill, a dam separates them. He installed a valve to empty the upper lake into the lower lake if needed.
They say there is a science to it, /forums/images/graemlins/grin.gif if I ever need one I'm calling him.