I think they should start Horford, Grant, Tatum, Brown, and Smart until Rob is back. I don't like moving your 2 best players around on the floor, such that I'd just keep Brown and Tatum as the starting SG and SF all season long (at least as much as possible). And I really don't want Tatum starting at PF. I think that would be too much strain on him. Parts of the game, him playing there is fine, but I don't think he should start there or play big minutes there. Keep Tatum at SF. That is his best position and you need to maximize your best player.
So in that scenario, yes Brogdon winning 6th man would be a real possibility.