I will try to play with camber angles, using some camber kit.
I want to put some -1 deg. Do you know if I have to change the caster angles accordingly, so as to work properly?
My car has the same " floor", as the S3, and the same suspension.
No, castor and camber are different, and have different effects on the handling.
You have to know what you want to achieve when playing with either...especially castor.
As for camber, you may be able to get -1.0 degree on the bottom balljoints without resorting to a camber/castor kit.
Another note, -1.0 degree negatve camber will be better than standard...but in my experience, it's not enough. Look to go nearer -1.5...possiblt a shade more to really improve things.
But, in answer to your original question, on an S3 (or similar)...No, you don't need to adjust castor to change camber.
But using a camber kit (assuming it's a top mount adjustable type) you may change the castor too...this may be unavoidable.
You can also considser optimising castor by moving the flont lower subframe about.