[quote=@FallenTrinity]
can a witcher also be a mage or are they combat mages who wield swords as well?
[/quote]

Witchers can cast basic spells known as 'Signs' only. Five of them are Aard, Igni, Quen, Axii and Yrden.

Any other form of spells are Sorcerer/Sorceress exclusive.