They redesigned the Malibu in 2008. Based on the pictures I've seen of the older Malibus, compared with my husband's 2008 Malibu, I believe that the new backseat isn't as roomy as the old backseat. I'd say use the Turbobooster and the Scenera, because those would give you the best chances of fitting an adult in the back, too. I think the True Fit is probably too wide to work with in the back with another car seat and an adult (I've had mine rear-facing in the center and forward-facing behind the driver), though it might work with the Turbobooster. I hear the Nautilus is pretty wide, as well. Hopefully, I'm wrong, and you'll be able to work with any combo you want or they'll overlap well enough next to each other to leave room for an adult.
And just a little FYI quirk: if you do a car seat in the center, it will definitely have to be with the seatbelt, although there is a center LATCH. If you use LATCH in the center, then you can't use the driver's side outboard position next to it for either LATCH or seatbelt installs or an adult passenger.