Is it possible to have a cluster made up of servers of different versions? For instance, could I add a 6.1.9 server to a cluster of 5.2.5 servers?
No. FDB is protocol compatible only within patch versions (so the
x.y.z). A 5.2 server will refuse to communicate with a 6.1 server, but a 6.1.0 server will communicate with a 6.1.9 server.