I cleaned out an old pond that was originally about a half acre and enlarged it to an acre in 2013. Not exactly but very similar to what you are doing.
I also had a small amount of water left in the old pond. There appeared to be minnows in the small puddle left but did not see anything else. Turns out there were green sunfish and bullheads, both of which caused me to abandon my original plans for the fish I was going to stock and are a source of ongoing management problems.
Here is my saga of leaving some of the old water.
danger of leaving some old water So I would echo one of the recommendations already given. Kill out whatever is left in your old pond and start fresh stocking what fish varieties and sizes you want. Don't be limited by what fish you already have in the old portion of the pond. I had certain nostalgic ideas of leaving some of the fish that came from a pond over 50 years old that I grew up with as a kid. The nostalgia was misplaced. If I had it to do over, I would have killed all of what was there and started fresh.
That is my suggestion. Looks like you will have a beautiful pond and you and your grandson will get great pleasure from it. Good luck with it and welcome to PBF.
