1) When can I introduce treats/ scraps to my chicks?
One of the first things my broody hens do with her chicks is take them to a patch of dirt so they can peck at the ground and get some grit. Then she takes them where they can eat grass and other stuff. So a broody hen starts feeding her chicks treats immediately. As long as they have grit they can have treats immediately. But you are not a broody hen, you are feeding them what they eat instead of taking them to forage for it. About 90% of what they eat should be chick feed, not treats, so they maintain a balanced diet.
2) My 11 week old black sexlinks are eating constantly; can I let them go without food for a stretch?
I don't know what you mean by "a stretch". It's generally recommended that food be available while they are awake. They tend to eat to fill up their crop then don't eat until the crop empties, then they fill up again. I'm not sure what you hope to accomplish by restricting feed. Personally I would not do it.
They do not eat at night in the dark so food does not have to be available them but I'd have it available during the day.
3) How old do they have to be to stay outside permanently in SE Louisiana?
I lived 32 years in south Louisiana. This time of year it is hot, including at night. From a temperature aspect they can go outside at 4 weeks. Three weeks would probably be OK. I often have mine outside with no supplemental heat at 5 weeks even with the overnight temperatures below freezing.
Predators are a different issue. Unless you have them in a secure place they are at risk even when they are fully grown. For most predators size or age doesn't matter that much. Dogs, coyotes, foxes, raccoons, and many other critters will take a full sized chicken. Birds of prey like hawks and owls will take a full sized chicken and eat it on the spot, but smaller ones at more at risk, especially with the smaller hawks and owls. A snake will eat a chicken, but it depends on the size of the snake and the size of the chicken. It would take a really big snake to eat an 11 week old chick, you are not likely to see a snake that big though I've seen some big ones in the marshes and swamps down there when fishing.
Predators are hard to talk about. Some people totally free range their chickens and just don't have issues with predators. Some people get wiped out as soon as they try. The only safe way is to have good barriers to keep the chickens contained and predators out. The stronger your barriers the safer the chickens but stronger barriers take more work and are more expensive.