Hi,
Personally I find the option to only rely on your time machine backup scary. Yes it is supposed to work and yes it usually does, but if it does not.. your options are limited.
If you intend on installing El Capitan, why not install it on an external firewire drive? That way you can preserve your working setup, at least for now and until the new setup has been known to work well.
VMware Fusion has an internal file sharing mechanism that you can use. So basically you can use any file location on your El Capitan host and share it with your virtual machine, this does not need a network card and works reasonably well. I'm careful with my wording on that as it is reliable for normal use.
But if you need for example high performance then you're probably better of by using conventional file sharing techniques like an NFS or AFP share.
I'm not exactly sure on how well your planned usage (Final Cut Pro 7) is a fit for a VM.
For two reasons:
The guest display adapter support in an OS X guest OS is pretty poor, it works, but there's no hardware accelerated openGL support.
That's the reason I would expect HD videos to be problematic...
Unfortunately there's way better adapters on Linux and Windows guests.
This is a very sad state, considering that the product runs on OS X itself.
I run OS X guests here also and regularly see screen artifacts due to the above.
The second reason is that video editing tends to be processor intensive and isn't the best workload for a virtual machine.
You normally want to run that on as many cores as you have available and a VM can't use all cores of the host as it will starve the host OS (and thus also the guest OS)
So you will loose at least a few cores.
Experience tells me that normally you do not want to assign more as half the cores on a host (real cores, hyper-threading cores do not count)
--
Wil