The stock sizes in a Saab 900 are 4 " under the front grills with room for a one inch tweeter above the demist vent and 4m" or 6X4r" under the rear parcel shelf supports. If your rear supports are made for 4t" speaker you can't fit 6X4's and vice versa, 6X4's are generally capable of being better than 4e" speakers but can over power the front sound stage (i.e. you hear sound coming from the back rather than all around you) generally this isn't a problem as all head units have a fader control.
Rear Speakers: Your only choice here is co-axels due to the space, well that's not quite true but unless you want to start chopping things up they are. If you have 6X4 speakers there then Kenwood, Rockford Fosgate, Sony or JBL are the best people to speak to, if you have 4t" speakers then the information from the front speakers guide applies.