Actually, Shane is wrong about your computer.
You have the new cylinder MP, and it does not have 4 drive slots. Nor can you boot in any OS earlier than what your computer had when launched so you are limited to 10.9 or later. OS boot is always restricted to the original shipped OS or later.
Since you were successfully using FCP7, prior to updating, you might try to restoring back to the last OS version you were using. It sounds like you have some conflict with the latest update. What OS and version were you using??
If you are having crashes, then you can look in your log files/Diagnostic Reports to see what process is causing the actual crash. It could be any number of things including 3rd party apps/plug-ins or hardware. Even things like Fonts can cause a problem. You may have a corrupt file inside FCP7 causing the issue. If you have Motion created files in your project, this can cause the crash too.
Would help to have all the info so we can help to troubleshoot the issues.
As to using FCP7 in El Capitan, it will work, but you have to expect some issues. As Shane says, this is no longer being supported. Apple is changing the underlying frameworks that run your computer and editing programs. Soon, very soon, legacy FCP won't run at all. I really suggest you consider FCPX and transfer any projects from FCP7 to 10 using the excellent 7toX app. Try to finish your current project in FCP7 if you can.
Personally, I love the new FCPX and hate if I have to spend any time in FCP7.
Sorry you are having the issues, that is very stressful.
Hope this helps, Greg