You've got the number of speakers right. Ford counts the mid and tweater in the components as seperate speakers so markets the Shaker 500 a 8 speaker system. The sizes of the factory door and rear speakers are actually 5x7 but 6x8s will fit in the enclosures (6x9's however will NOT). The rear speakers aren't exactly woofers only as Crutchfields website suggest but a dual cone system. You'll get a bit more than base only but nothing compared to what you'd hear with a quality 2 or 3 way speaker receiving a full range signal. The factory grill doesn't come off the speaker and its a little hard seeing down there but it appears to be paper cone to me. They are 4ohm, 25 watts. The component speakers don't have a wattage rating on their backs unless it is 130 (the number is on the back but no W). I wouldn't be surprised if only mid and high frequencies are going to the door speakers as originally you had only the mid and tweater components plus you have the subs there as well. I still have my subs running although as I understand it most people that replace the factory head unit and amp loose the door subs.
Due to the factory crossover that is at work here, 2-ways certainly would suffice for the rear replacements unless you are totally going to replace the head unit and amps. Eventually I will but I haven't gotten around to it.
Mine has a Rockford Fosgate amp and 10" punch 2 sub added to the trunk to help out on the base.
The "A" Shaker 500 head unit sounded better than the "B" version does. The "B" is alot more muddier with the base. Replacing the factory speakers with the 4-ways was a good step but they are hindered by the fact they aren't receiving full range signals.
Unfortunately to put in a quality 4 channel amp to feed the door and rear speakers, a 1 farad capacitor to handle the power of the 2 amps, and rewire the entire car to get around their damn crossovers it will run a good bit
Earl