Rosetta under Yosemite?
There is no way at all to run Rosetta in Yosemite. Rosetta was included in Mac OS X 10.4.4–10.6.8 (Intel) only.

The Apple license agreement doesn't allow customers to run a Snow Leopard client  in a virtual environment.
So the only legal way to use PPC apps with Yosemite is indeed Snow Leopard Server (Apple part number is MC588Z/A internal link (It only works in their store)) in Parallels (10.0 recommended) or VMware Fusion (at least 6.0.5, 7.0 recommended).