Hi, we have two BAsis Admin. for around 25 instances and 12 servers (3500 users)
To make the monitoring use the cluster transactions. You can check all servers and instances to same time. Sample SM50 is local process, SM66 is cluster application server process.
For us the main problem is the support to heterogeneous application servers.
We have logon groups for load balancing. some groups over hp-ux servers and others over VM-Ware with Linux SUSE operating systems.
Use additional tools check management engine for SAP.
Regards.