I am not that worried about "rules" but I think a flat-front shirt looks better with a three-piece tux.
I agree, but I don't think pleats look bad. This one I definitely chalk up to preference. I was just contemplating the other day where a pleated shirt would fall on my list to buy. I do think a flat front is the next one I'll get, but can't decide on a covered placket, or studs.
It's a daytime wedding outdoors with evening reception. Main issue is having too much variance in shades and I guess therefore formality.