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...I'm thinking M mode for aperture/shutter, but ETTL so flash will be the variable.
That's the way I usually set up for events where I'm moving around a dance floor or tracking subjects on a stage, for example, but for a fixed location with different subjects coming into a constant scene I'd use manual flash as well as manual exposure. That way the overall lighting, background tone, etc. won't change when one guy walks in wearing a tan jacket and the next comes in wearing a dark blue one, or one guy is a pale-skinned Irish-American and the next is a dark-skinned African-American; both situations could throw off the ETTL meter.
I have a flash meter, but lacking that you could take test shots of a gray card before the subjects start arriving, and adjust power until your histogram shows a spike in the center. (If you don't yet have a large gray card, you can take a white bath towel or pillow case and adjust flash power until the spike is just below clipping on the right side of the histogram...but a gray card looks more professional.)
...If you had a choice, would you choose a dark background or a light one?
For what you're doing, generally a middle-toned one. If the foyer doesn't have an interesting background -- like a bookcase, trophy case, photo wall, etc. -- then I'd make or buy a neutral BG in shades of blues, grays, browns, and greens. Here's an example:
The reversible BGs have a lighter side and a darker side, and when you're doing a one-light portrait without a seperate background light(s), I'd use the lighter side to compensate for the fall-off from the single light on the BG versus the subject.
With the subject a few feet from the BG, and the light raised over the camera on a bracket or stand, the head shadow will fall below the bottom of the frame, or at least low enough not to be a serious distraction.