You did not mention which version of Final Cut Pro you want to install in El Capitan. The following information is with regard to Final Cut Pro 6, Final Cut Studio 2 and its cousin Final Cut Express 3.5:
Apple has created a "bug" in the attempt to install FCP6 in Lion, Mt. Lion, Mavericks, Yosemite and El Capitan, which makes it look like Rosetta is needed for a PowerPC app, the installer, so as to make it difficult for the use of FCP6 in these versions of OS X! There are three ways to install it:
1. Pacifist https://www.charlessoft.com/
2. Jeremy Johnstone's Terminal method http://www.jeremyjohnstone.com/blog/2012-03-11-installing-final-cut-pro-studio-2 -0-on-mac-os-x-10-7-lion.html
3. HawaiianHippie has discovered the best and safest possible solution to this problem:
https://discussions.apple.com/thread/5591321?answerId=26309669022#26309669022&ac_cid=tw123456#26309669
I have now successfully done this on four computers, brand new iMacs running 10.9.4. And I've discovered an even easier way to do this! You don't need to edit the plist file, just move it into the Trash. Install proceeds normally. You can return it after you're done if you like, I don't know what (if any) possible repercussions might come from leaving it deleted.
The file, as indicated on Jeremy Johnstone's blog, is
/System/Library/CoreServices/Installer.app/Contents/Resources/PPCRequirements.pl ist
(Note that when you get to Installer.app, you will have to Control+click and select "Show Package Contents")
I have successfully duplicated the HawaiianHippie method on Yosemite and, after turning off Gatekeeper and SIP, it also works in El Capitan (and then after full installation and upgrade of FCP, be sure to turn these security features back on).
So, in response to Meg The Dog's comment "Also, FCS2 has problems with its included installer, requiring Rosetta to run (which is no longer included in the Operating System), so it won't install without a workaround to that." -- As shown by Jeremy Johnstone's research, linked above, Rosetta, or its lack thereof, has nothing to do with the installation of FCP 6.
In response to K Shaffer's suggestion about a hoped for running FCP in Snow Leopard Server in virtualization, I have tried it in Parallels and the limitations of the emulated video card and VRAM cause FCP 6 to fail to install and run. Perhaps the results would be different in VMWare Fusion or VirtualHub.