May 2018
Intermediate to advanced
576 pages
30h 25m
English
The preceding list doesn't actually say this explicitly, but you should use the same major version of PostgreSQL for all systems. With PSR, you are required to do that, so it doesn't even need to be said.
I've heard people argue that it's OK to have dissimilar systems and even that it's a good idea because if you get a bug, it only affects one node. I'd say that the massive increase in complexity is much more likely to cause problems.