You can use melee freely even as a mage (and you will probably have to, sometimes), but you won't be (canonically) able to learn two-handed weapons mastery. However, there is a way how you can be a Fire Mage AND a guard... temporarily.
---
Tell Thorus you're interested in the path of magic. Go to the Corristo, tell him you want to become a mage and answer the questions but DON'T go upstairs with him (to take the oath). Now go to Thorus and become a guard. Get Light Guard armor + Guard Sword from Stone.
Now you can learn crossbows from Scorpio and 2H from Thorus (but this will cost you 70LP and you have to be a 1H master already). After that, go to Corristo and take the oath. But beware, from now on you're a mage and your membership with the guards is over! That means Thorus won't teach you anymore and Scorpio only from 4th chapter onwards. Continue the game, playing as a mage.
---
This way, you can learn even 2H mastery as Fire Mage, but you must have the total of 100LP that you'll put in 1H/2H.
