As long as they are not so advanced that they surpass the rest of the ranged weapons, I'm perfectly fine with guns being common in the game. Or, if they are more advanced than the rest, then they should be uncommon, or require a very specific set of skills to be used efficiently, kind of a prestige class weapon.
A XVII-XVIII century like level of technological advancement (but with magic, of course), it's perfectly fine with me. Also, if you are having a background with different races and cultures clashing, one of the variables that decides the results of that kind of conflicts is the technological advancement (not only in weaponry, but also in the production areas of society, the spreading of technology sometimes goes hand to hand with aculturation), sometimes accomplished trough circunstancial environmental advantages.
So, if the game depicts cultural clashing, variable levels of technology (and weapons) are logical to me; but of course they must not unbalance the gameplay.
