I think the answer is "it depends"....As long as you don't use an features in versions > 12 I think (but have not tested) that you should probably be OK mixing the two versions (and that you have the setting in FM to allow access for 12).
I would run a test on a single machine using a test .FMP12 file so I don't risk corrupting any data.
And, see what others here say as well.
I would also call FileMaker's 800 number if one of the "TS" Folks don't respond here to make sure.
HOPE THIS HELPS.
So far as I know the new security features in FMS 15 that prevent v12 clients from connecting are not an issue in P2P environment. Should work fine.