On Magic Users and Multi-classing

Ever since reading about Gandalf weilding the magical sword Glamdring, I've always wondered why mages can't use swords. I suppose I've understood the argument about too much metal, but I thought it was kind of silly, if the iconic magiciain in literature can carry a sword. So, I'm thinking of lifting the restriction on edged weapons for mages in my campaign. They still can't wear metal armour - that much cold iron around their body will mess up magic's ability to function - but I'm thinking of allowing mages to use swords of a size up to a longsword without penalty. However, the trade off is, they don't get to use their STR damage bonus with any weapons, unless they take a level as a fighter somewhere down the road. I've also been thinking about the idea I saw on a blog (I really need to start archinving this stuff when I find it) about magic-users being able to counter spells. If cold iron is what hinders magic, then a magic-user using an iron sword to help disrupt the casting process of another mage makes sense to me.

I've also been thinking about my homebrew mutli-classing rules. I'm thinking of stipulating that players can't multi-class until they hit level 4. In other words, they have to stay with the class they start with for 3 levels. When they hit 4th level, then they can start to branch out with other classes.