Hey bro … that gap has puzzled more than one new n old beekeeper ! . Looks like couple people have you filled in here.
Don’t sweat the space … Your right space but not wide enough for a nineth frame. Don’t even try that. Just take your hive tools n squeeze the 8 frames to the center leave no wide gaps between them … Centering leaves equal gap at each outer wall frame. Bees don’t seem side or bridge comb those two outer gaps. So not an issue.
Curious about your frames … I know the Nuc frames are filled out. Did you get a very mature (extreme full), average or lean Nuc ? And are you using foundation or foundationless outer frames.
Just a thought … if foundationless I’d might slide the Nuc frame with food/honey to the outside giving your girls a guide one both side for at least one side. Not sure your Nucs would be strong enough to do on both outer sides. That might be pushing their bee nunber recsourse.
If those extra frames (lighter color/New) are foundationless … inspect about once a week to check n correct creative honeycomb building. Using this sandwich method I’ve been able to greatly reduce or stop much sideways comb bullding which can be a pain or cause lots of grief down the line trying to maintain!
Here’s several pix’s n a drawing for you ! Hopefully these are helpful.
Good luck n Happy beekeeping.