Couple Tub Surround Questions

First question: I saw a nice surround and it said something like "installs directly to studs". Does that mean I can't use it if I already have drywall installed, or does that mean it can be used either way but has the advantage of being able to be installed directly to the studs if you desire?

Second question: I've read a little and what I read indicates that a five piece surround is better to use in situations where your walls are not 100% squared. One of the walls is, but the other is not. How do I decide how many pieces to use? Are 5 piece surrounds more likely to possibly have leaks because there are more seams as opposed to a one or three piece?

TIA!

Each seam is a potential leak point...

The surround walls probably have a flange that is designed to be covered by the drywall that butts up to it. It adds stability and strength. It would look lousy sitting on top of your drywall, so you might just want to tear the drywall off, square up and plumb the walls, install the new surround, then replace the drywall needed to cover things up.

To square up the walls, you could sister new studs next to what you have, or add shims. You might also be able to plane them, but that takes more skill.

Five part surrounds probably have corner pieces that secure to the surface of the wall panels. If so, the will ALWAYS be a potential leak source, since it can be difficult to keep them adhered to the walls.