I have done this fairly often - bring in tracks that were overdubed somewhere else (say, on a 20 bit ADAT) into a 24 bit project. It doesn't hurt anything at all.
Sure, the overall project would arguably be "better" if everything was done using the best gear possible for the whole thing - this means the best bit resolution, best converters, optimum preamp and front end signal path (mics, compressor, etc) but I think the most important thing is the MUSIC. If you have something that was done on an ADAT that just FEELS great, and you have not been able to duplicate that feel again on the "better" gear, then by all means you should not hesitate to use it.
I've heard very well done recordings in 16 bits and some pretty bad sounding 24 bit recordings. I prefer 24 bit, but I hear a greater difference between 16 and 20 bit recordings than I hear between 20 and 24 bit recordings, all other things being equal.
Again, there's really no reason NOT to fly in and use a lower bit recording if it suits and serves the music. The effect and results of higher or lower bit rates add up with each added track - they're cumulative, so you may indeed hear a difference if you're using lots of tracks from 20 or 16 bits, but if they are well recorded and again, more importantly, FEEL great, IMO, it's worth it to use them.
An example: We had a lead vocal that we did here on a Telefunken ELA M 251, and it sounded great... but the guy just couldn't get the same feel on the bridge that he had at home on his ADAT with a SM58 when he demoed the song. I flew the bridge in from the ADAT, and then I had to try to match the tone of the '58 / 16 bit ADAT track and the Telefunken / 24 bit HDR track... I think I did, and I would be willing to bet money that if I gave you a copy of the album and let you try to guess which song I did this on, you couldn't tell me which one it was. I would have preferred it if I could have gotten the feel here in the studio (it would have saved me some work) but it turned out fine in the end.
Sound Sanctuary Recording
Riverside CA http://www.ssrstudio.com