One solution that might be easier and no more cost than cleaning out the old ponds is just build a new small forage pond at the same time your contractor builds your main pond. Lot simpler, will cost some money but maybe no more than renovating the old ponds, and will work like you want from the get go.
My forage pond thread Speacialty pond list Right now I have only FHM in my 1/20th acre forage pond (with plans to grow out some HSB fry next year) after it was killed off this spring. When I feed there are FHM thick.
My experience is if you put FHM and GSH in the same small pond, the FHM will eventually be extirpated. The GSH out compete the FHM. In a larger pond might not be a problem, but in my 1/20th acre forage pond I had lots of FHM but they only lasted about a year after I introduced GSH. Your results might be different.
