Hi
Probably the first thing to do would be check on the legal aspect. Koi /Goldfish can be quite invasive in lots of situations . I suspect the Koi would do WAAYYY too well lol gary

There are probably too many ways for the Koi and any other fish to migrate in and out of the lagoon. The koi would quickly become prey or predator. Introduction of non native species rarely has a good outcome. The DNR would be the place to check.

Conjecture here: near the Chattahoochie you are in largemouth bass country? Bass rather like to eat goldfish and they can consume some rather remarkably large ones. My uncle had a farm pond, quarter acre perhaps, and he tried goldfish several times. Bass would somehow get into the pond and the goldfish would disappear.

Yes, Koi would probably do well.

But it probably would be breaking all sorts of laws.

If there are no large predators there, that would indicate there is not much worth catching in the lagoon.

Gators, otters, herons etc will soon move in when there is nice large sushi to find

It is illegal to release non native fish into a natural body of water in Georgia.

If its subject to huricanes and near salt water and the pond can be floded with salt water you will lose your koi, iI lost ten 24 to 30 inch koi some pinecone buterfly when the salt water got in the pond and floded my koi out LOL Hoitider

Please don't.

Here in Massachusetts we have a huge problem with people dumping unwanted pet koi in local ponds. They quickly destroy the native species and take over. I suspect it may be even worse where you are.

Please, don't do it.