Mixing the sources is not the issue at all. I have mixed various sources of different bit depth and sample rates without dithering or resampling without issue at all. That's quite easy to do in Sony Vegas.
My question was mainly aimed at the finished product rendering essentially being limited to the lowest sample rate and bit depth of the various sources.
my Blue Turtle Seduction show in September last year was done this way (24 aud, 16 sbd). You still have 24 full bits in the end product if you render it as such, because (in theory) you are including information from the last 8 bits of the 24bit aud source. Can you
realistically hear them in the final mix after bringing in the board? Doubtful, but
it's possible, and if you can, they are there.
It's not as good (on paper) as 24 and 24, but 24 and 16 still can have 24bit information, just not much of it.