It sounds to me like it's going to be the way you play the character more than the level they're at that makes the difference. I'm in a long-running D&D campaign (almost at the end of our fourth year) and in that time, we've had npc's with us who were higher and lower level than our party. We had a paladin npc with us who was a higher level than the rest of us when he joined our group (he stayed with us for about a year in-and-out of game time)... of course, we leveled up far faster than he did, so by the time we left him, we were about two levels higher than him. But he didn't hinder us, or our ability to be the heroes in any way. In fact, on one occasion, he was the reason we got in trouble in the first place!

I don't think there'd be anything wrong with your idea as long as you're careful about the way you play the character, and it already sounds like you've got the idea that you want this person to be support only, so it doesn't sound like you've got too much to worry about.