My novice thoughts are...If you have a heavy population of GSF already, experimenting with crappie and perch can't make it any worse.
I'm not privy to yellow perch info, but putting crappie in the pond is kinda like putting in more GSF, other than the potential for a large fish if things work out for you. The both can overpopulate a small BOW rather quickly.
I would certainly do more investigation as to what is actually in the pond before stocking any thing else. You may have better luck killing the pond off and starting fresh.
Then again, a lot of GSF can be a nice start to a catfish pond or even growing some nice bass.
As far as GSF controlling the Crappie and perch YOY, it sounds like they, the GSF, have plenty to eat now with all the fry and minnows. I doubt that adding small crappie or perch forage would mean much to them if forage is abundant already.
I'm just thinking out loud, just my 2 cents worth if that!
