Question

MAC Pro 2008, OSX El Capitan, 2 X Quad Core Xeon @ 2.8 Ghz, 32 GB RAM @ 667 Mhz DDR2, Nvidia Geforce GT 512 MB

Answer 1

So long as your interface is compatible with your computer you should be able to use it inside of Studio One.

As for Studio One 3 Artist being compatible with your machine you can see the system requirements for Studio One here:

http://www.presonus.com/products/Studio-One/tech-specs

Based on the minimum requirements you should be able to run Studio One 3 on your machine.