They will fight initially, but that is perfectly normal.
Any established cat will make sure the newcomer knows who is top cat, so there will be a degree of spitting and chasing until they settle down who is who between them, and then all will be peace - apart from the occasional spat when one feels like being a bit bossy.
If your current cat is not that old, he will probably accept a cat of either gender, but you will need to get him 'done' if you buy a queen, in fact, if you have a tom, get both of them neutored, cuts down on the machismo factor which always causes fights.