Besides Red Rocket Truckstop?

Still on my loner playthrough, but I'd like to move somewhere new.

5 off the top of my head Jamaica Plain, the drive in, a construction site in a swamp, Croup Manor and then that island. (Name escapes me at the moment.)

For the drive in I'm pretty sure you have to set up beacon when you first unlock it (at least I already have), so there are already people there.

I'll check into the others.

Only after you've unlocked it but since you've unlocked it you don't have to. At least I didn't.

quite a few, also I mentioned beacons in another thread, you can take them down immediately after you place them and the quest will complete and you should get no settlers, and if you do simply send them to your actual settlement.