I'm doing a Smokies trip in September too. Clingman's Dome is the highest elevation, but there are other similar elevations across the AT and the other northeastern ridge. I would look at temps between what you see there and in Gatlinburg, Tn.
Generally, September is pretty nice. I plan on bringing a 30 degree quilt. I would definitely take your rain gear including your skirt. If you're in the Smokies very long, you're likely to see rain. I will probably bring a light puffy, but I think a fleece would be fine too.