Muscovy are very self reliant. I grew up in Florida and I can tell you that they did just fine as feral animals so much so that they are now killed by the state.
As long as they have access to a large area to forage that offers a wide variety of bugs, plants, and a water source, I think you should be fine. The only thing that you may run into is lower egg production if they are not able to find enough of a protein source.
They will breed but you may want to keep track of how many eggs they lay and how many ducklings hatch out and compare that to the average (which varies for scovies) and see if supplementing feed would help.
I am encouraging mine to free range. Right now, I feed 5 geese, 5 ducks and 9 chickens (all between 4-7 months old) one 2-qt scoop of feed twice a day. Not all have started laying yet since the days are short but I am getting 5-6 eggs per day from the chickens.