A couple of points.
First, remember that you can always use a reducer to make a larger fitting into a smaller one, but you can not go the other way around.
Second, it takes a long time for water to come out through small openings. I recommend going to 3" for gray water unless your tank is smaller than 45 gallons. Also, don't forget low-point drains on the fresh tanks.
1.5" is fine for most wet-vented waste stacks unless you have more than three fixtures. That's also fine for the black vent.
We added extra fittings on the other side of the tanks, plus 4" openings top-center in case we ever needed to clean them out. PVC pipe plugs are cheap, so you can't have too many fittings, only too few.
We also added low and high 1/2" fittings on the two fresh tanks and the gray tank for sight glasses, which have come in handy many times.