Changing bit depth in a session

[vap80]

Hi,
I have a question about bit depths in a session. I always want to check the wav and mp3 exports of my sessions. I use 32 bit floating in composing ,mixing and mastering stage. Then I export to 24bit wav (16bit if it is for cd) and mp3 (16bit).

According to manual "You can change the bit depth for all newly recorded or imported (and converted) files in the session by selecting a different Bit Depth option in the Session Setup window."

I suppose a session can handle multiple bit depths, but I cannot find anything on that in the manual or internet (maybe I do not search correctly ). So I just want to ensure that when changing the session's bit depth while working on them it keeps the old clips in the initial bit depth and imports the new files in the changed bit depth.

I always want to check metering to the new exports with izotope insight 2. I do that mostly to check the mp3 version as it is the most common type of file in digital music world, so I want to be as good as possible. Also I'd like to ask what kind of conversion does the Pro Tools to an imported mp3 file. Does it affect the sound?