Yes, you can use the head unit to power the 6x9's. Granted at most you are only going to get 20-25 watts into each, but no problems running them off the HU.
Good suggestion there. Before spending on an amp for the 6x9's I would try them out first on the Hu and see what you think about rear fill. Many people have very different opinions on it. If you like the fill, then def. amp them, it will brighten the sound tremendously for you.
depending on what deck is it, it might not even come with a hp/lp filter. your rears will sound shitty at certain volumes. but this is not a common problem on the better decks on there.