Those Memphis are not OEM. Probably from the previous owner.
You might want to read about the AFBS (Acoustic Feed-Back System). There is a little black box under the passenger rear speaker, that's the AFBS brain/amp. It is only connected to the rear speakers.
I don't think it will damage your speakers, but the AFBS is designed to run 2 ohm rated speakers (the OEM ones) while most aftermarket speakers (including yours) are rated for 4 ohms. If you use an aftermarket head unit, it could potentially get damaged by the AFBS (this I'm not sure about). The front OEM speakers are rated at 4 ohms and not connected to the AFBS.
Also, the feedback part of the system might not work well with your speakers, possibly causing distorted sounding output. And, the AFBS combines the stereo signal into mono, meaning your rear speakers are in mono not stereo.
It is easy to bypass the AFBS. You just have to cut and splice the correct wires in the speaker wire harness before the AFBS. I don't know which wires it is off the top of my head. Try searching, it may have been posted here.
Or, you could just live with it and deal with it.
I forgot...the Polk dB series of 6x9's work well. Mine fit fine, but the magnet is a bit big and hits the trunk release spring when I shut it. Not really a problem though. You can find them on ebay or other online retailers for under $100/pair.
Found a good link:
Prelude Audio