Bounce to disk/dither question

[Gothboy]

I'm collaborating with a vocalist right now and we'll be swapping our ideas back and forth via email until we're able to be in the same room together for the final tracking of her vocals of this particular song. She's using Garageband and I'm using PT 6.4 and 5.1.1. I don't know which version of GB she uses yet but I'm pretty sure it only imports and records 16-bit audio. Only the brand new '08 version of GB does 24-bit. GB won't import a multi-track PT project either so I have to send her a 2-track .aiff or Mp3 file of the song. Once she tracks her guide vocal she'll export her tracks as .aiff files and send them to me. I'll then import/convert them into my project. I'm pretty sure I have to dither my song from 24 to 16 as I'm recording my stuff in 24 bit. I've got all my tracks recorded and mixed in 6.4 so I now have to bounce to disk and dither as well, right? I've looked at the manual and I understand the basics of this but how does this apply to my situation?
Since I'm not mastering/burning to disk is it just a matter of creating a stereo master fader and applying the dither plug? How do I route all the tracks to this master fader and what options should I use for the bounce? The manual points out a ton of bouncing options and they're a little confusing.
My plan was to send her a zipped .aiff file but then I thought maybe it should be an MP3 because it would be a smaller file. Can anyone give me a simple, detailed run-through of the process or point me to a thread that explains it well?
If there is something I might have missed please fill me in, I want to get this right the first time.

[M.Brane]

While it is a good idea to dither whenever you go down in bitrate you don't really have to if it's something that will not see final release. Depending on the material you may not even notice a difference.

You can import 16bit files into a 24bit session with the convert & import command in the region bin menu. It will just add 8 empty bits to the file so no penalty in sound.

[Phil Ogden]

For overdubbing vocals, a mp3 is fine (provided your singer can handle the import of this format into GB). I've done this many times without problems.

No routing necessary for the Master Fader. Simply create a stereo master and choose mp3 (if you have this option) from the BTD menu. Otherwise bounce as a 16-bit/44.1Khz AIFF and use another compression utility like iTunes.

[Sebastien Richard]

I second the mp3 method.
A quick note is keep things simple by using an agreed upon song length so that as you guys transfer files back and forth they import to a predetermined location in your protoolPro Toolss session ie. no work to get the timing right.

[Gothboy]

I ended up bouncing to disk using the 'stereo interleaved', convert during bounce and .aiff options since I don't have the direct to Mp3 option in PT, then I imported the song into Audacity and converted it to an Mp3. Worked great,sounds fine... but I noticed that the master faders showed that there was some clipping going on during the bounce and when I imported into Audacity the file was hitting the red a bit so I used a master limiter effect in Audacity to tame it a bit. Should my master faders not show any clipping? It's hard to tell what's going on in the bounce process since you can't do anything when the bounce is going on.

[M.Brane]

It's not a good idea to mix that hot. You might consider dropping your mix level globally to keep the master out of the red. You could also simply put a look-ahead limiter on the master with the output set to -1db as a security blanket. Once you clip in digital anything you do farther down the chain is moot.

I hate having to constantly worry about clipping while mixing so I mix with plenty of headroom on the master then make up any leftover when the mix is done. That way I can concentrate on making the mix sound good instead of just loud.