The 1985 Fiero uses 4X10 in the dash and a pair of 3.5" in the seats.
As for the subwoofer in the center console, They make a kit to fit an 8" subwoofer. A 10" is too large because you don't have enough box volume for one.
Here is a place that sells the kit:
(add the www in front of the link.) clubredline.net/tlgauto/index.php?page=shop.product_details&product_id=19&flypage=flypage.tpl&pop=0&option=com_virtuemart&Itemid=11
For the dash:
The Kicker 08KS4100, Pioneer TSA4103, or Kenwood KFC-415C are your best choices. Unless you want to go to a 5.25" woofer with seperate tweeter. In my car I used high quality 4" with a seperate tweeter.
For the seats:
Kicker 08KS350, Rockford Fosgate P132, or Infinity Reference 3032cf.
It is possible to fit 4" coaxial speakers in the seats as well if you want to upgrade them.
I have a 1986 which has the rear speakers in the sail panels so I used 4"" with a seperate tweeter.
You can see pics of my system here:
(add www to the link.) polkaudio.com/caraudio/installs/view.php?id=13