I apologize, I misread your initial post. I assumed you were advising the OP not to shoot multiple frame rates.
Are you sure FCP doesn’t mix frame rates on the timeline? I could’ve sworn the last time I used FCP that we had mixed frame rates. I suppose I was mistaken.
Surely there is a way to do this in Avid.
I’m running into the same problems with footage from a P2 card. A couple of action shots were at 60 for slow-mo but the rest is 24.