Yes the 66-16 and 66-18 have a back tack. I tend to forget about those two models. By all means do clean up the 66 and 99. They will serve you well for many tasks.
I've done a couple of small upholstery projects with my domestics and it can be a struggle with thick seams. I think you will be happier with an industrial with a walking foot for upholstery work. Watch out for your fingers with a walking foot, you quickly learn how powerful the industrials are. It will bring tears to your eyes.