You definitely don't need FCP to get Compressor.
But you do need Yosemite. Apple does make available updates to earlier versions of applications for people running 10.7 and higher, but AFAIK it's only for those who have previously purchased the application. Here is the support doc if you're interested in testing whether it will work for a new purchase.
Alternatively, if you don't want to update 10 10, there are a couple of dual boot methods to consider: 1) You may be able to partition your existing drive and install 10.10 on that volume. 2) Connect an external drive and install 10.10 on that/ In either case, you would download Compressor and boot from the new volume when you wanted to run the application.
Russ