TT Multi Scale Layout

I am working the design for the new layout as I am moving. My current layout is an HOn3 that was to built in three sections. The radius was to be 16 1/2' for the loop part, which would have been too tight for my locomotive (MDC 2-8-0). I enjoy narrow gauge but TTn42 is so close in  gauge (1.5mm) that it wasn't worth it to me when a reasonable amount of stuff is available in HOn3. But I love TT scale and well, even HO is too big to do what I want. So..., what to do? Eureka! I have decided to model a multi scale layout! One half will be TT and the other half of the 4x8 will be HOn3. The HOn3 will be higher in elevation. The TT side will have a staging yard under the HOn3 half and the HOn3 tracks will be in the back of the TT section. They are a mm and a half smaller and will offer a bit of forced perspective. I just won't operate them at the same time. I am building the continuos loop first and not just the yard like my previous layout. Just looking at the preliminary design I can see you fit a lot in in 1/120 scale. I was so used to the constant compromise in HO. Every time I got an idea I would have  to adjust. TT is great,  as I can "fit" stuff in and it is a good feeling. I have one other thing going  for the layout as I built the track layout in HO in the 80's/90's and I know what will work. That layout was an eleven foot shelf layout. I just have to bend it slightly at each end then use curved turnouts on one end and add an oil refinery (partially on the backdrop) and it's good to go. I want to get a quick start on the layout, so I am going to use commercial track (Tillig. It looks great!), then relay it later with code 55 hand laid track and Fast Tracks kits. N Scale cork roadbed works well in HOn3 so I think I'll use it for the TT. Of course, because of the design I'll have to scratch build the structures, but I  have to do that in TT Scale anyway and the HO buildings were heavily kit bashed from DPM kits.  Although Walthers has a nice N Scale Printers building that can be "raised" with a concrete foundation and scale doors. Although it seems to be easier to  "cut-down" HO buildings.