JD,
For years I used both FasTrack and Conventional Tubular Lionel Track & Switches on my bigger multi level layouts, it was very convenient before FasTrack Command Control Switches, for me to use old Conventional 711/072 and 022 Lionel switches, it also opened some design engineering doors, on my layout, space wise. Transitioning in and out of my old Conventional Track & Switches was no big deal with the FT transition pieces, and as you are planning to do, I used most of the O Gauge Conventional Track in places where it was usually not readily seen, and very few people even realized I was transitioning in and out of the old conventional tubular switches, everything running with DCS. All in all it worked out fine and saved me serious money on not purchasing the Lionel FasTrack 1st Generation switches. I have to admit wiring my older 711/072 Lionel Switches thru the DCS AIU's, with all the wiring was not the way I wanted to go, however at that time it was the best there was for remote control switches, of course after Lionel invented the FasTrack Command Control switches, I also invested in Legacy for operating full wireless remote control switches. This Lionel engineering is IMO the real reason to purchase the Legacy/TMCC package. I eliminated the DCS AIU and all the wiring that goes with it, sold some of my old 072 and 022 Lionel Conventional switches, and added the Lionel wireless Command Control FasTrack Switches, to upgrade the layout. Blending the different type O gauge Track & Switches together to make the multi-level layout was no big engineering feat, just go slow and do it correctly and your custom built layout will run perfectly.
PCRR/Dave