Add everything but the girls first. If you add the girls first, they'll claim the tank as their territory and will not take kindly to anything else invading it. Your first stocking idea (tetras, then cat and pleco, then girls) is the best.
Yes, you can bump the number of girls up to 8 with no problems. As I said, I kept nine of them in an 18gal with no problems (again, wouldn't recommend it, but that's what I did).