Hey all. Does an HA harvester run workloads on management nodes? Or are those sacrificed compute if too large?

Is there a setting required to allow hosting vms?

No - if you have 1 or 3 nodes in your Harvester cluster (i.e. they're all management nodes - nodes with management role) then they should host VMs. If you have 2 or 4+ nodes you might find the non-management nodes are favoured but you can migrate VMs to the management nodes.