Assuming linear scaling, there would be a line missing in step #3 of the Setup section:
"6 Players = 8 Hub Planets"
I would definitely suggest that don't use this and instead you use your second suggestion of
"6 Players = 7 Hub Planets"
so that as you say at least one Imperial Fleet starts on the board.
I also used the same idea of giving the 6th player the same number of starting rebels as the 5th player rather than incrementing it.