I’d kind of expect the belt to keep the idlers in line, but I know Ryan recommends the 10mm idlers even with the 6mm belts because 1)they fit the trucks and 2)apparently they tend to have better bearings.
Either way, since this isn’t the thing holding you up, I think you’ll feel a lot better about having the bigger belt, especially later.
Speaking of bigger belts later, now that I’m in my forties, I find my old belts have to be upsized, too.