Well, a Soldier 1 won't outclass a Barbarian 5. So no fear. The only type of multiclassing I don't like is when someone takes a level in a second or third class merely for benefits (multiclassing to the 3.0 Ranger, ferrinstance). But as this would fit with the concept, I'd have no problem with multiclassing for this guy.
Just FYI - right now, the PCs are in a fight with some Zamoran bountyhunters, who after the party thief (who I've been NPCing, I dunno if his player's still around). Soon, they'll wander into the Valley of Toads. I figure they'll find you outside the valley.