My 4 acre pond has sago pondweed and coontail taking it over. I'm giong to treat it chemically but after that is cleared out, I'd like to find an alternative plant to replace it.

My idea is to plant a very short growing grass/weed that just grows a few inches instead of 6 feet long like coontail. That way, it will outcompete the coontail and keep it from growing back again.

Are there any pond/aquatic grasses/weeds that will suit this purpose?

I'm going to stock with grass carp and tilapia as well but I think using competing plants would be a good option as well

Sago pondweed is a good cultivar and while it can get dense it responds readily to herbicides and isn't hard to manage in my experience. Coontail - obviously a different story. I'd be cautious about nuking the pond and leaving a clean slate - you can have dense planktonic algae blooms or FA explode. If you can selectively treat the coontail I'd do so and target the sago only in areas you need clearing [around a feeder, dock, whatever] and leave the rest so you don't have so many free nutrients for the algae to get going. Lots of beneficial macrophytes - some of the forum favorites are american and illinois pondweed, corkscrew eelgrass, and emergents like sedges, rushes, arrowhead and of course hybrid lilies. I would not clear the pond THEN stock GC - you could be inviting some turbidity issues to arise if the pond is devoid of forage for the GC.