What you say is more or less right. At 1st level, the first encounter - adapted from Night's Dark Terror (and I think upthread I have said The Night Below when I mean Night's Dark Terror - getting my "Nights" confused!) - involved the PCs on a boat being stopped by a chain across the river and then swarmed by cultists. I statted the cultists as 1st level minions (delevelled rabble). Just recently, at 12th level, the PCs were attacked in the streets outside the apartments of a wizard they had fought and killed, and some of the assailants were thugs from the docks. I statted these as (I think) 10th level minions, levelled up from the minions in the Monster Vault. (Which I think are human thugs.)
OK... I know that Night's Dark Terror encounter with the swimming Iron Ring slavers AIR - I think I'd probably have statted them as standard 1st level monsters (brute or skirmisher probably), if there are more than 7 or 8 then maybe make the rest of them Human Rabble min-2. And I'd make the dock thugs as 9th or 10th level minions same as you did, thus both (standard-1s and min-9s) would have about the same XPV and in-world the same threat level. I like how flexible 4e is that way.