I would allow 1.25" per cardinal.
Putting you at:
2 x Platy
6 x Cardinal
1 x Balloon Molly
So I have you at more like 15" But if you wanted to add another molly or another platy then that would be fine.
Don't get me wrong, if you look at John89, their tank probably wont encounter any filtering issues and may not have any aggression from the gouramis (though I'd be very wary of it).
But, being on the 'safe' side of stocking isn't a bad idea for your first tank.
So if the above is your potential beginning stock then what would you like too add after the tank has mature a bit?
If you add real plants and find that your tap water has a reasonable level of Nitrates then I'd advise you could go up to 28" of fish (after the first 6 months).
So if you had
2 x Platy
6 x Cardinal
1 x Balloon Molly
Then you'd have another 13" of fish to play with.
I would definitely suggest some bottom feeders. Ideas: Pygmy Corys, Normal Corys, Kuhli Loach.