I would not recommend this approach.  The APC (or any plate carrier for that matter) is designed to carry plates, not act as a soft armor carrier.  I understand what you are thinking, but plates that fit your requirement (threat type, and then select a size for your body) should come next..  If stand alone or ICW it matters not, depends on your requirements.  Only after that should one select a carrier that fits that particular combination.  Using a PC for soft armor only is a no-go.  

You are already at the APC (great choice), so the assumption is you require Large or XL plates.  If you go 10x12 the plates will move around a good bit in the plate pocket.  Sure, some do that but the Sm/Med carrier would be in order for those.  Remember the carrier is sized for the plates, the cummerbund is sized for your waist (along last rib is what I have my customers measure).  Either way, plates need to come next in the equation, along with backers (if required).  As far as sides you are tracking, plates are optional for most.  Soft armor sides for the APC are 5x11 for your carrier if you have a large cummerbund.  The 6x8 soft panel that you mentioned will not work properly in that carrier, .  
Hope this helps.

vr
Mike

AT Armor SCARAB Carrier Giveaway!!!!