Well, welcome to the world of the misinformation. Or is that disinformation? Either way, the best way to know is to actually look and hope it's not a mirage. Ok. The rears have room for 4" AND 6x9's. Yep! There are 4 spots for speakers. 2 for 4" and 2 for 6x9's. Not sure if the rear deck could hold up all the weight, but you could put 4 speakers back there! A jammin' machine on wheels! But wait! How do you power that much speaker? That's where I will be "splicing" in my amps.
And YES! Disconnect the odd center speaker. It's sucking power from the rest. The door speakers are 6.5" speakers. There should be no worries for space in the doors or rear deck. I haven't upgraded my doors, yet. Now, I have a '97 900 that I wanted to upgrade as well. It worked out because the amp in that system was integrated in the stereo unit. More easily bypassed. When I "splice" mine in, I have to be aware of which speakers I will be amping. In the '97, it powered the rears only, I believe. In this generation of stereo system, I believe it powers the doors and the rears.
I know this went afar from what you were asking, but you will be underwhelmed when you want more volume. So, I wanted to address the option.
There is a link here that gives a schematic of the different stereo systems available in this generation of Saabs. Then grab your infotainment manual and check out where things are located. Also, Frank could answer some of your questions better than I, I'm sure. Let's hope he's watching.

Don't be embarrassed by your questions. We have all asked them and some are still looking for definitive answers. They exist... somewhere in this world. AND... Crutchfield is very informative but not much help when it comes to Saabs. 10:1 they were reaching for the 10' pole when they heard you mention Saab.
Saba’s ES2 150/7 Stereo Upgrade, OR 'How To Add Aftermarket Amps/Speakers to '03-'06 - SaabCentral Forums Try this link for some decent info.
Bladwijzers