If you are going to go with Fi Q, I would definitely go as big as you can fit.
The Q is one of those lines with almost identical motors on each model.
and the price difference between the 10" and the 18" is only $60.
so your gaining a huge amount of efficiency relatively inexpesively... personally i wouldn't buy a 10" Q if i can fit a bigger model, it would make almost no sense.
Also if you wanted to run 2 subs you can still go with the FI's...
You don't need 2 dual 4 ohm's to get to 1 ohm, The Q's come in dual 1's which can reach 1 ohm by series I would recommend parallel on the VC's and series the subs, rather than the other way around.
I would go with 1 big sub the 18 will be much louder than 2 10's off your amp and be far less expensive, not to mention the wow factor of having a 18" wanger