Might want to ask this on a ProTools forum as well.
fwiw I have a 4 core MacMini Server w 8GB RAM and a 2008 MacPro 8 core w 16GB.
The Mini is used as a general file and A/V server when not specificially tasked with Final Cut Suite efforts.
Unless I have serious Compressor processes running (e.g. converting h.264 to an editable format) that draws on the full cores, the mini can do a good job keeping up with the MacPro.
What the mini won't do is take 3rd party cards or drive multiple large monitors but for digital audio it ought to be fine with thunderbolt drives.