Why would you need to record separate tracks at different bit depths? If the sources are a different bit depth then Audition will record them all as 32 bit but with the unused bits set to 0. All your subsequent processing will be in 32 bit anyway.
My initial aim was to limit the size of files. I also wanted to see if Audition could do it. (Apparently it can't.) More recently still, I've been recording three tracks simultaneously at 32 bit and 88200 KHz just to see how my machine handles it. The file sizes are relatively huge, but I want to see if I can detect a smoothness to the higher sample rates that isn't there at CD 16-bit/44.1 KHz. I bet I won't be able to tell the difference. But that's another issue.
I highly doubt you'll hear any difference with higher sample rates. And I'm sure you won't hear any difference
recording at 16 or 32bitFP. Processing quality, however, will likely be another story...
Emmett