installing 6 speakers on 4 channels

i'm an installer at Circuit City, but have only been doing it about 2 months, so I still consider myself a newb.  Anyway, this question is about my own car, a 93 talon.  I want to replace all the speakers in the car(dash, front door, back hatch area, but my concern is with the front door and dash speakers.  I'm worried if I install aftermarket speakers in these locations that I'll cause an impedence problem on the front channels.  Won't installing new 4 ohm speakers in the door and also in the dash present a 2 ohm load to the head unit?  The head unit is a alpine 9811.  I'm guessing the stock speakers in the front and dash are 8 ohn to give a satisfactory 4 ohm impedence.  Is installing components the only way to wire this correctly?

I already have alpine coax speakers ready to go in the doors but am holding off until I'm sure I won't cause a problem with the head unit.

You are correct, if you wire 2 sets of 4 ohm speakers to the front you will get a 2 ohm load on the head unit which it cannot handle. Either leave the speakers out of the dash, or get a small 2 channel amp for the fronts that can handle 2 ohms X 2. You will want to wire both sets of speakers in parallel which will give a 2 ohm load on the amp. I would suggest you go with the amp you will get better sound then if you were to use your head units built in amplifier to drive them. You could also get a 4 channel and amplify all of your speakers and wire your 2 front sets in parallel.

You probably have tweeters in your dash if anything is there.  In this case if you have coaxial speakers to replace them with ignore the tweeter.  That is what I did in my riviera.  However the best way is to use a four channel amp to power the speakers if you are installing 2 sets.  This way you power them better and you will get a better sound.