@Randolph_Carter Welcome to the forum.
Can you add nodes to a Couchbase cluster if they reside on slightly different hardware?
The nodes can be on different hardware, there are many cases where different hardware profile would be of benefit for the different Couchbase services . For example most of the time the Query service does not use disks, so there is no point in giving it fast SSDs. For more information see documentation - MDS.
We have three nodes in an ‘old’ couchbase cluster on Cisco B200 M4 servers. We would like to add 2 more nodes, but can only purchase M5 servers.
Generally we recommend that all the nodes that are running a service have the same hardware profile for ease of management and for debug-ability . In this case it’s fine to expand the cluster using the newer M5 servers, please ensure that the new servers have the same amount of RAM and can performance as good as the older ones.