2011/2013 is a long time ago, and I'm sure FCP and Logic have come a long way since then.
I agree with what you're saying: just make it simple and install them along with a current, compatible OSX (well
MacOS, we've come full circle again which I tend to forget all the time) on an external SSD.
I would have to replace my graphic card in my Mac Pro (mid-2010) with a Metal compatible one, but as far as I know that's not a huge financial investment, neither is an external SSD as you say. Actually, I have two SSDs (Samsung 830, 128 GB each) already installed in my Mac Pro. I used one of them as a Photoshop scratch disk, but has now got MacOS 10.11 El Capitan and my apps on it (as I recently upgraded from 10.9.5 Mavericks which is on the other SSD along with the apps for it). In a few weeks I'm sure I can say the transition/test period is over and I can safely erase that Mavericks SSD and use it for whatever I like
The only downside to this is I won't be able to use Photoshop CS4 on the fly (I would have to reboot into the older OS first), or all my other apps on the fly (if I've booted into the more recent OS with FCP/Logic on it). I'm not sure if Lightroom (perpetual) will work with more recent 64-bit only OSes either. Those are the two apps I can't/won't upgrade (it's all monthly fees now from Adobe).
Unless.... there's a way to run Photoshop and Lightroom in a virtual environment within the newer OS. Or else I just have to plan ahead and do a FCP or Logic session and nothing else before I reboot into my usual OS for all my other apps.
I guess it's not very smart to share files between two OSes (i.e. running Mail, Safari, Firefox etc. from either OS I boot into, accessing the same email message archive, web bookmarks etc. as in the other oS as
Fishrrman pointed out. That won't be an issue if I install ONLY FCP and Logic on that newer OS' SSD of course, but will it be OK to save the project files by those apps on my regular HDD? Obviously they won't be accessed by any app within the older OS, but I heard something about a new file system and wonder if that also rules out saving/accessing files stored on the "old" file system?