I was thinking of getting a 20 degree quilt for use in the smokies during oct/nov.
To be more versatile, could I get a 30 or 40 degree quilt and sleep in a mont bell inner down jacket? That way I would have a lighter quilt for warmer weather.

It really depends on your weather forecast, which can vary quite a bit that time of year. Also, lower elevations might be fine while higher elevations would not. I would want at least a 30 degree quilt that time of year, even with supplemental clothing. I have been quite a bit in Nov. and had temps of low 30s, but it can get quite a bit cooler too.

Well maybe I should go 20 degree and a down jacket jet to be safe. I was just trying to shave onces

Using the down inner as part of your sleep system is a good idea, but it's not that warm of a jacket so it won't be enough to turn a 30-40F quilt into a 20F one. At most it would give you about 5 degrees....I'd really guess about 3. I find I can get about 10 degrees max (ie. 35-40F quilt down to about 25-30F) with a much warmer Montbell Alpine Light Parka and similarly warm GooseFeet Down pants.

So if you expect lows around 25-30F, and thus want to bring a 20F quilt to have a margin of safety - by utilizing your MB Down Inner you could instead bring a ~25F quilt and still have that margin of safety with the down inner.

Are you using a bivy or a double wall tent? Without those, I'd say the jacket will be needed just to keep the 30 degree quilt a 30 degree quilt due to drafts. :)

Consider the insulation underneath you also. What keeps you warm at 40 degrees may not be warm enough at 20 degrees, even with the jacket and a 20 degree quilt.