Questions about pathfinder 1e games
I'm currently playing Pathfinder for Eberron and Gamma World, among other games. It works well for any setting that D&D uses (and you can just port over Product Identity if you need to use cool stuff like illithids).
I think that "one side must be single-classed" is something people do, I'm not convinced that it actually adds anything. Then again, I single-class in Pathfinder much more frequently than I did in 3.5, despite multiclassing being easier than ever. (In part because you don't have the situation where prestige classes are strictly better than the base class - Sorcerer, I'm looking at you!)
As for hybrids, they aren't an issue, and I wish the "hybrid class" designation had never been born. A hybrid class, to a greater or lesser extent, is designed to do its own thing even if it uses spell lists and mechanics from other classes.