As far as the ohms/watts, etc- you'd be fine running those in parallel with the rear speakers in my opinion.
Having said that, I would just not even bother for two reasons.
1. Your goal should be to have better/louder/cleaner sound from your front speakers- you don't go to a concert and put your back to the stage, do you?
2. I will put money on it that those speakers will sound like dog **** in your truck, if you were even able to hear them much at all over the stock speakers.
I'm not trying to bash you at all, so don't take it the wrong way! I'm just trying to save you some grief. Don't bother unless you really just have time to kill and want to hear the results for yourself. But don't say I didn't warn you!
When you are able to save up a few bucks, check out sonicelectronix.com (yes spelled just like that). You can get a damn nice set of Infinity Kappa 6X8's for around $100-120. Then a decent 2 channel amp that puts out say, 50 watts/channel @ 4 ohms and you'd be set up as far as front stage speakers. (The Kappa's are 2 ohm speakers, so that amp figure example would realistically give them about 80 watts RMS)
From there you could either run a sub from bridging the amp (most 2 channel amps will run a set of speakers AND be able to be bridged at the same time) OR just run another decent amp off the rear channel outputs of the stereo into a sub or two. In all, even buying new stuff you could get a decent little set up for not much money. Buying USED amps will save you some bucks if you want a big one for the sub. The door speakers won't need that much power to be LOUD. Believe me, even at 20 watts of REAL power, it will be loud. The stock deck (and most aftermarket for that matter) only put out a FEW watts realistically.
Good luck man.