I always start a new mix by importing session data from a previous song on the same project. Usually you can at least count on your drums and bass to be ballpark of what they need to be. Guitars/synths/vocals/etc usually change too much between songs to rely on importing sounds from another.
As for keeping all the songs in the same session, It is a little more taxing on your computer (processing wise) but if it can handle it, then its fine. I know a lot of guys who do this.