Thanks for checking. I still have vendors that use it, and still need it to open old projects on occasion. We moved over to PP a few years back and dont regret it. We've tried a couple projects in FCPX and just not fan.
Looking back, I wish we always stayed on Avid and never made the move to FCP 7.
Are you trying to install it with the disc's or dmg files? If you're trying to use the disc's, then I would make all the disc's into dmg files. Mount all 7 dmg's and then install it that way. Using the dmg's works almost 99% because the disc's for some reason are programmed with application software that won't work on anything above mountain lion.
Out of curious, why upgrade to El Capitan?
I keep a partition containing a stable/pervious OS. When a new OS is released, I install the a newer OS on a second partition or drive just for software testing. I haven't messed with El Capitan yet.