Hi Sian
We use a breathable on all surfaces, gravel, grass, soil, anything you like. They're very tough and shouldn't tear. We use chairs and tables on top of it on gravel pitches and have never had a problem. You can get various grades, so possibly avoid thinner cheaper ones.
Can't really see the point of putting a breathable over a none breathable groundsheet or other material, this would rather defeat the object, other than you'd get more padding under foot. The idea of a breathable on a grass pitch is so that you don't kill the grass. We lift ours after a week and can hardly see where it's been. Anyway, why carry two around instead of just the one.
Nah, we find a breathable is good on any pitch, and as you don't always know what type of pitch you'll end up on, it's easier just to bring the one.