I suppose it would kinda depend on what model of speakers you are using for the other channels. Perhaps it's not so critical in a vehicle, but I know that most people who have 5.1 like to have matching speakers with the same consistent timbre. And, is that correct, a 1.5" opening? How exactly are you planning to mount the center speaker?
As for the 4 ohm vs 8 ohm bit, go with the 4 ohm speakers. You'll get more out of your amp. Since your using an all-in-one type of amp, I would also imagine that your amp would be happier if you had a consistent load for all the channels rather than a mix and match assortment of speakers and resistances.
Don't forget, combat pay is most useful when you come home safe and sound. Good Luck