First of all, I must say that I've never shot anything similar. That said, what I'd do is, based on your initial ideas, choose the Fuji Xtra 400 as film. That would allow you to get a lot of background without extremely long exposure speeds. Then, knowing your flash GN, set the power to a medium value to illuminate people within your desired range. And fingers crossed, as always :-)
AFAIK, the higher the film sensitivity, the better adaptability to lighting conditions. I mean, Portra 160 is not as flexible as Portra 400.
I have no examples. Sorry. But this is what I would do, or at least, that would be my first thought.
Good luck!