Spoilers ahead that you may want to be aware of for your playthrough;

• Regarding your question about the legendary raiders: In Nuka World, you will encounter three raider clans that may or may not make a presense in the Commonwealth. Once you capture a Minutemen settlement for the Nuka Raiders, Nuka Raiders will start to appear in the Commonwealth in random encounters. You will encounter a lot of legendary raiders that way. As long as you're allied with the Nuka Raiders, they will be friendly to you. You can, of course, pickpocket them whenever you see fit.
• >!Capturing Commonwealth settlements for the raiders will earn you Overboss perks. You need to capture 8 settlements to get all perks. Raider settlements work in a different way than Minutemen settlements and you will gain access to new resources. Here's what you need to know about them if you're a completionist;
• Raiders have their own versions of stores. You will need to unlock them through one of the Overboss perks. The Raiders will have a somewhat different store inventory than Minutemen settlers BUT * They can be assigned to Minutemen settlers' stores and have the same inventory as when theyre assigned to the Raider versions of the stores. * If you kill all raiders in a settlement and regain control of it for the Minutemen, your Minutemen settlers can man Raider stores and they will function just the same as regular stores.
• In raider settlements you can build Booze Stills. They require one man and will generate happiness as well as a steady supply of booze in your workshop. Once a raider settlement falls in Minutemen hands again, Minutemen settler CAN operate the Booze Stills and generate booze.
• In a similar manner, once you've unlocked all Overboss perks you'll be able to build one Tribute Chest per raider settlement. These will generate random loot over time and make you extremely rich. They can also generate Legendaries so with these babies in your settlements, you will effectively turn them in Legendary farms. You can only build them in raider settlements but Minutemen settlers CAN operate them once the settlement is back in their hands.
• All other resources that can be built in raider settlements will continue to operate in Minutemen hands, like the chem dispensing fridges. Raider Radio will go silent some time after completing Open Season, however.
• All the above will mean that in the end, Minutemen settlements will have more potential if you've captured them for the raiders, built raider resources and if you capture the settlements back later on. !<

Once Open Season has been completed, Preston will talk to you once again. However, if you've captured a Minutemen settlement at any point during your Nuka World playthrough, you will lose him as a follower unless you have never met him until after completing Nuka World. For a 'perfect' playthrough, you will need to complete Nuka World before you meet him in Concord.

I'm probably one of the few who loves raider settlements. If you have any questions about em, I can pretty much tell you every detail about how they work.