I have done that, even added five nodes that contribute a separate Gluster file system
using dispersed (erasure codes, more efficient) mode.
But in another cluster with such a 3-node-HCI base, I had a lot (3 or 4) of compute nodes,
that were actually dual-boot or just shut off when not used: Even used the GUI to do that
This caused strange issues as I shut down all three compute-only nodes: Gluster reported
loss of quorum, and essentially the entire HCI lost storage, even if these compute nodes
didn't add bricks to the Gluster at all. In fact the compute nodes probably
shouldn't have even participated in the Gluster, since they were only clients, but the
Cockpit wizard added them anyway.
I believe this is because HCI is designed to support adding extra nodes in sets of three
e.g. for a 9-node setup, which should be really nice with 7+2 disperse encoding.
I didn't dare reproduce the situation intentionally, but if you should come across
this, perhaps you can document and report it. If the (most of) extra nodes are permanently
running, you don't need to worry.
In terms of regaining control, you mostly have to make sure you turn the missing nodes
back on, oVirt can be astonishingly resilient. If you then remove the nodes prior to
shutdown, the quorum issue goes away.