It will run fine on 8 cells. You just have to be careful not to overprop for the voltage because current is not longer the limiting factor, heat is. The motor will overheat before current will tell you you have too much prop.
You won't need anywhere near the power it can produce on 8 cells so this won't be a big issue if flying in a reasonably scale manner.
When doing series packs you must have the same C rating and the same capacity. Its better to have packs that are the same age and have essentially the same usage history.
I avoid using more than a 4S pack because all of my higher cell count packs have had the middle cells fail early. The middle cells have a harder time getting rid of heat. If the packs have to be stuffed in with no airspace between its important to get them out and separated so they can cool after a flight.