Need Advice on "consolidating" computers

I’m looking to consolidate our two computers and wanted to get some comments on the proper way to go about it. I’ve been complacent with the setup for too many years and now it’s getting long in the tooth.

Current setup:

Mac Pro Aluminum Monster (Late 2006)

250 GB Internal SATA (10.6.8)

500 GB Internal SATA (BootCamp Vista)

1.5 TB Internal SATA (Time Machine)

21.5 inch iMac (Mid 2010)

500GB Internal SATA (Partitioned 250GB - 10.9.5 / 250GB - BootCamp Vista)

The Mac Pro has pretty much reached the end of it’s life. The fans run on high at all times, the screen goes black if I attempt to play any game (fullscreen or windowed), something either graphics or logic board related, that isn't worth the cost to fix. I can’t upgrade past 10.6.8 and Adobe CS products have really started to lag on it.The iMac has basically been a light use backup and Word/Email machine…

What I’d like to do it pull the drives out of the Mac Pro and them in External SATA Enclosures to be bootable from the iMac and designate them as follows:

500 GB Internal - Install Yosemite

500 GB in External Case - Move existing Mac Pro system with 10.6.8 intact, I have several "legacy" programs that are not compatible with 10.9 or 10.10 so I need to keep an old system running

250 GB in External Case - Time Machine

1.5TB in External Case - Is it possible to setup as an always on Network Drive to basically move my iTunes library to so that I can access via networked xbox 360 even when the computer is not on?