Actually Final Cut 6/7 is NOT a PowerPC app. Mountain Lion supports it. Folks are having mixed results with running the installer in ML. But most find the installer runs just find.
ML will run 32-bit apps just fine. Just not apps with 32-bit Kernel Extensions, which FCP 6/7 do not have. But they are not "PowerPC" apps at all. They are fully Intel apps.
I highly recommend getting proper training for FCP X. macProVideo.com has the most affordable and extensive training videos on the market. They have tons of free short written tutorials on their HUB section. Second best is Ripple Training. Everything else is garbage, IMHO.
I'm running FCP X 10.0.5 (soon to be 10.0.6 we hope) on a 2008 Mac Pro, 2007 17" MBP, and a 15" rMBP. No problems on any of them.
Word of warning: It's GPU and RAM hungry, so be sure your rMBP is maxed out on RAM, and has the largest SSD drive. You'll also need an external (secondary) drive for Events and Projects. Do not run them on the OS drive. FW800 (via adapter) or T'bolt (out performs Mac Pros) will do you just fine.
But take a course in person or online and save yourself a ton of trial and error (mostly errors) and dispell the myths right up front. It's very, very worth the money and time.
Tons of free user made plugins (and 99% are extreme pro quality) at fcp.co on the Free Templates forum there.
I'll never go back to FCP 7, I'll never go back to PPro, I'll never waste time on Avid MC. FCP X is all that, and more.