Factory installed front speakers are 10 ohm. Rear speakers are 4 ohm. This was done primarily as the design of the factory radio incorporates two IC power amplifiers, one for the right channel, one for the left, so that each chip is loaded with the same impedance. Sound level is typically biased heavy to the rear in car audio, so the lower impedance drivers in back give some emphasis in the sound level.
You'll be hard pressed to find aftermarket speakers that are 10 ohm. Those were made special for Delco to GM specs. Most everything in auto sound made now is 4 ohms or less.The 1988-91 models used the same speakers in the doors. There are 5.25" woofers on the door panels, down low behind cloth grille, and a component tweeter up high behind slotted plastic grille. I recommend replacing the woofer and leaving the factory tweeter (and crossover) hooked up, even if you install a 5.25" coaxial or triaxial speaker in the woofer position. If you disconnect the high mount tweeter, imaging and high frequency response will be poor.
The stock rear speaker setup for 1988 and '89 cars is a 6.5" woofer and a component tweeter, the latter being nearly identical to the one used on the front doors. Since the rear speakers on coupes mounted to a plastic adapter baffle, the 6x9 setup could be transplanted into the 88/89 cars easily enough by getting the baffles out of a 90/91. I've done that in my 88's as a 6x9 provides better low end response than a 6.5" woofer due to the increased surface area of the cone. There are also many 3-way (a.k.a. triaxial) 6x9's available that incorporate a midrange and tweeter as well, for better response across the entire frequency range than is provided by a two way (woofer/tweeter) combo.
The 1990 and '91 rear speakers are common 6"x9" type mounted to a plastic adapter panel behind the grille. A good coaxial or triaxial speaker system is recommended, and there are many to choose from.
On convertibles (1990-91), the front speaker complement is the same as on coupes. The rear speakers are 4"x10". There are a few models available in this size, but don't expect overwhelming results as a cone that size cannot generate much low end, so your bass response will bottom out around 60-90hz, leaving it sounding a little thin. Good bass performance needs to extend to at least 35Hz, and preferably as low as 20-25 to do a good job of accurately reproducing most music.
If you want really good sound in a convertible, a slimline powered subwoofer (Kicker makes a decent one that isn't obscenely expensive) is pretty well required IMHO, even with new 4x10's installed.