I have a question regarding etiquette in the transition phase between tracking and mixing, and dealing with clients in my own studio.
I recently had a session where a band I was working with had finished tracking on several songs. Before we finished up some of the other tracks, the band gave me permission to to go ahead and pre mix some of the tracks that I had thought were done. I proceeded to mix the songs and put plugins into the mix. When the band came back in they surprisingly said they had vocals to add to one of the mixed tracks. I explained to them that my system (running dual cores on a macbook pro) wouldn't be able to handle it with all the plug ins I've already added, even with low latency within my DAW (Logic 9), and the latency would be too much for them to hear the mix correctly. They understood, and I handled it as professionally as I could, and they decided to opt against doing the extra vocal tracks.
My question is: would the proper way to approach this in the studio be to go on as I did and skip adding tracks after the tracks have been mixed? Or is it more typical to strip the entire session back down to allow for the client to add for their additional vocal track?
In many cases you do need to go back and record extra parts or re-record solos for one reason or another, so you need to be prepared for this. In your case it appears your system was a bit undersized for what you were trying to achieve.
You may have been able to solve this by printing the rough mix to a wav file and adding it as another track. Disable all the other tracks and plugins and have the extra parts record against your printed rough mix.
For the long term, however, you probably need to evaluate your system needs, and maybe get a box with a lot more grunt that can run the mix in realtime with the plugins and record extra parts. Perhaps adding a DSP box for some plugins (eg UAD firewire DSP box) may help as well.