There are instructions on apples site regarding fcpx and fcp7 on the same machine. That is much trickier than avid & fcp.
Fcpx doesn’t use QuickTime. It uses av foundation. So they don’t interfere with each other. I would say the safest would be fcpx & avid on the same machine. But I do have all three on one of our machines and there haven’t been any issues yet.
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