Tyre recommendation
 

First of all, I ride a "Ghost" mtb, a decent 26er with these rims and these tyres. However, I'm riding 70% on trails / roads and 30% pure off road cycling, so I would like to have some slimmer, all-round tyres for my bike. On Mavic website, recommended tyre widths for my bike are 2.10 to 2.50, although I would go even lower, 2.0 would feel perfect since my last bike had that size, but I'm not sure how safe that is.

Anyway, any recommendation is welcome.:)

Have you thought about buying an alternate set of wheels or at least an alternate front wheel? If you did, you could have a set of knobbies and a set of slick, thin road tires.
As for going to 2.0, I doubt that going to a slightly smaller width would be a problem, I wouldn't hesitate to make that change.

N+1 Is always best..

Where N= the # Of bikes you currently have :P

I like the extra wheel set advice the most, I swap tires now and then so I stick with my tubed setup...

But for road rides I have another bike, a Recumbent.