As far as speakers go, I would look for nice components online and a pair of door sails at the junkyard that had the tweeters so you can run a wire up the door to a tweeter. It will be exactly the same as the 6 speaker system in the Alero.
I would also recommend if you can to upgrade those old 4x6 to 6 1/2s on the door. I did it with mine. This is only if you can come up with a way to mount them. I bought an adapter online, but then again, not sure if the Malibu had the speakers mounted the same way the Alero/Grand Am did.
As far as the rear deck, a pair of Sony 3 way speakers will suffice. I have a set in my car. 60 watts RMS, peak is 300. While the radio will never match it (remember radios usually supply 20 watts RMS on average), it still sounds pretty clear and can play bass lines really good.
Here are the speakers I bought. Well worth the money.
http://www.bestbuy.com/site/Sony+-+6...&skuId=1982098