If I record tracks at 16 bit and add some 24 bit audio (like say from ADD2), does it "hurt" the 16 bit tracks when I dither the whole mix? Or should I just dither the drums separately and return them to the project at 16 bit where I can burn them all without dithering everything?

Only dither when you reduce bit depth.

I don't think you understood my question. I understand only to dither when reducing bit depth but that wasn't the question.

Dynochrome
If I record tracks at 16 bit and add some 24 bit audio (like say from ADD2), does it "hurt" the 16 bit tracks when I dither the whole mix? Or should I just dither the drums separately and return them to the project at 16 bit where I can burn them all without dithering everything?

No it won't "hurt" it, in fact could only help it. Any time audio is summed up and/or there are gain or processing changes, you should re-dither. Just mix as you would and then export as one with dither. 

Just to be clear,I'm talking about a project that has thirty 16 bit tracks and  one 24 bit track. If Dither "bothered" or negatively affected the 16 bit tracks at all. If it did, that't what I asked about dithering the 1 track separately,then reintroducing it to the project to export as is. I don't want to do that if it doesn't negatively affect the 16 bit tracks.
 

Dynochrome
Just to be clear,I'm talking about a project that has thirty 16 bit tracks and  one 24 bit track. If Dither "bothered" or negatively affected the 16 bit tracks at all. If it did, that't what I asked about dithering the 1 track separately,then reintroducing it to the project to export as is. I don't want to do that if it doesn't negatively affect the 16 bit tracks.
 

Dynochrome, see above response.

I'm not entirely sure off the top of my head, but if your project is 16bit and you import a 24bit audio file it should get converted to 16bit on import. I think it will also be dithered according to the settings in preferences. So in that scenario, if you export the project to 16bit there is no need to dither. If the project is 24bit and you import a load of 16bit audio and one 24bit audio it will be the other way round.

35mm
I'm not entirely sure off the top of my head, but if your project is 16bit and you import a 24bit audio file it should get converted to 16bit on import. I think it will also be dithered according to the settings in preferences. So in that scenario, if you export the project to 16bit there is no need to dither. If the project is 24bit and you import a load of 16bit audio and one 24bit audio it will be the other way round.

The idea of dithering is not isolated to the file itself, it's not a black and white thing. You have to consider processing, gain stages, etc, and finally summing tracks. You are building word length, the final export, no matter the mixture of X-bit files, must be dithered down. 

In sonar, I don't think the project gets to be a global bit rate, I believe I've had projects with both 16 & 24 bit tracks on them.

Yea, but it's irrelevant anyway pending wordlength increases.