I have a very similiar setup in my 2007 vert. I have a Pioneer AVIC head unit that supplies the power to (4) Infinity reference 6x8s as well. The subs in your car are powered by a small amp each located in the kick panels. I recommend not using these and get a seperate 2 channel amp for the door subs. As for subs there is actually quite a bit of room (depth) in the door sub box for mounting. I went with JL audio 8" subs but there is plenty of others choices for subs that will fit. It may save some time to pull the door panel and get the depth dimension for mounting to assist with picking your subs.
07 GT vert, Satin Silver, 5spd, premium trim, interior upgrade pkg, comfort group, GT appearance package, cloth top
ADDED: 302 Stroker, KB 2.8 Mammoth @ 20psi, DOB belt drive, E85 fed, TR6060, ATI, Cervini hood, Meziere, Pypes Long Tubes, O/R X pipe, FRPP GTA mufflers, 20" Bullitts, Eibach Springs, BMR relo brackets/UCA/LCA`s, FRPP 4.10, etc, etc.