Earliest you can get Yuffie is after you clear the Midgar Zolom swamp. Enter the first forest you come to and start walking around in it. She should spawn within a few battles.
Pretty much my team right after leaving Midgar. Long Range is gained shortly after, so there's no need for Barret, while Tifa's chain limit outdoes anything Barret and Red can do. Besides, once you get the Powersoul, what's the point in ever taking her out of the party? Aerith is obvious for her magical prowess and limits.
Tifa having amazing expanding over the course of the game boobs and Aerith having wide, child birthing hips are just a bonus.
