Well, Twilight Alchemist, your last post was in July, so unless you come in and provide opinions, I think we should just alter this to suit the game best.
From your post, Star, we've kinda got the choice of scattering them to different dungeons all over the world, or keeping them in one place. I think it makes sense that if they're scattered all over the world, we'd have to order them in such a way that the item they steal, you actually have before you fight them. Not that this would be hard to do.
I think that the decision about whether to have it as an all-at-once battle vs a find one in each region sidequest should be made based primarily on the reward you get, and should also dictate somethings about their personality, like whether they're evil or just regular thieves.
This is all kinda scattered, but here's some random thoughts:
I like GM's idea of having them as Poes in the Gerudo desert or somewhere, who were killed during Ganon's reign, or alternatively, who died out after Ganon's reign. Suppose they were all haunting their old hideout in the desert, you could go in, have your items stolen, and then one you defeat them all, you'd get a cool reward like the Hurricane Spin move, or an upgrade to your pickpocketing speed.
We could also have them as a legendary band of thieves, who each guards one of the teleport hideout places. You'd go through a mini-dungeon to get to the boss room, they'd steal one of your items, then once you defeat them and prove your worth, they'd allow you to teleport to the hideout. Personally, I like the idea of the teleport hideouts as derelict and abandoned better, though.
Or they could even be a rival gang in Hyrule City. You'd gain access to their hideout after gaining all the items, and then you could go there, have your items stolen, defeat them all, and 1) they'd stop picking on your gang, and 2) you'd gain the use of some of their hideout, which could give you access to hidden places in Hyrule city, or shortcuts from one part of town to another. This method could also lend itself to having a sword attack learned or thief skill upgrade.
Or also, perhaps the 7 Sins could be old sword masters. You'd learn new sword moves by visiting their hideouts or whatever and upon defeating them, you'd be taught a sword move / thief skill.