Do you already own infant seats for your twins? If so, what are they?
The 3rd row is very narrow. This thread discusses a newer model:
http://www.car-seat.org/showthread.php?t=222331 that might give some ideas.
Instead of replacing the boosters for your older two I would consider replacing the Alpha Omega seat for your two year old. A Radian is very narrow and would provide more room than the current seat. At her age and size she could easily rear face (current recommendations are to rear face as long as possible), or forward face likely until booster readiness. Your third row has only one top tether, in the centre spot, so you'd be looking at putting the 2 year old there if she's forward-facing, installed with the seat belt.
That leaves the captain's chairs open for whatever infant seats you have, and just about anything should work there. The seats are very slanted however, so you'd want to make sure anything you do get allows the use of a rolled towel or pool noodle to increase the recline if needed. The one seat that comes to mind that does not allow that is a Chicco KeyFit. I don't know if it works on that seat with its own built in recline adjustment but it's something to try if you're shopping for them.
The other alternative is to put the babies in the 3rd row with either the 2 year old (rear or forward-facing) or one of the booster riders.
Given the tight space and slanted seats you're going to want to go and try hands on some seats. Babies R Us allows you to try the floor models, and do try installing rather than just sitting on the seat as the angles and fit can change quite a lot once a seat is installed.