Most freshwater founders end up being brackish water trinectus maculatus. May not go well with gullets as they maybe sensitive to salt.
Also if the flounders are from Peru then can be true fw but hard to say especially coming from a lfs. Likely a hogchoker.
Because a flounder can be foldable then I'd say no to it as a tankmate. They do come in at smaller sizes.
Gotta be 2.5 longer than the gulper itself to avoid being eaten.
Good tankmates for gulpers are other gulpers and driftwood. Hope that helps to make a decision for you!