We currently have a 6 indexer Splunk 5 cluster.
They're all currently running Splunk 5.0.4, with the exception of the universal forwarders, which are running 5.0.2.
We'd of course like to eventually get onto Splunk 6. I'm unsure about the order in which they should be upgraded though.
To experiment with some Splunk 6 features, I set up a copy on my local machine and tried to connect it to my existing Splunk 5 cluster master as a search head, but it seems the two are incompatible, so this leads me to believe there could be other incompatabilites between the two versions that I should probably know about..
Upgrade steps in short:
These steps are for 1 SH, 1 CM, 1 FWD, 4 IDX
1) set CM in maintenance mode
2) bring down all the peers
3) bring down the master
4) upgrade everybody - don't start! (untar over existing /splunk directory)
5) start master
6) set master in maintenance mode
7) start peers
8) start search-head
9) check things (searchability from SH, show cluster-bundle-status from CM)
10) if all is well, unset maintenance mode, then start the fwd
11) test fwd / inputs
Mix of 5.0.x and 6.0 are not supported. 6.0 search head makes additional REST requests from master, e.g. number of replicated copies, searchable copies of an index etc that are not supported on 5.0.x, thus not quite compatible. Would recommend you to follow ChrisG's suggestion about reading Upgrade a cluster topic.