Automotive Paint -What can't you mix? How long can it store?

Urethanes
Enamels
Acrylics
Lacquers
If I use oil based rustolium as a primer, what can I paint over it?
Por 15 and Lizard skin is expensive for us dollar scraping guys!

How risky is it to buy used paint or por 15 or other paint related chemicals used from craigslist -or even ebay? Can I mix similiar or dissimiliar chemical paints?
Is isopropyl (rubbing) alcohol good enough as a surface prep after sanding?

What do you use to clean the gun with when painting with the aforementioned chemicals? It's different from one paint to the next right? mineral spirits?