1. Use pull mode
yes Pull mode is always best.
2. Heartbeat set to 4 hours
it depends, for 75 machines, 2-3 hour could be the one i can think of.
3. Enable randomization
yes
4. Use a 15-minute period randomization window
yes it is ok .
About 5-6 servers (physical, virtual) monitored and about 40-75 PCs monitored, servers of course are always on and PCs will be off/on at different times. The SEP server itself is a virtual server.
SEP server on VM is supported. the client numbers shuggested here around 80.
You can configure the GUP in the network to reduce little load on the SEP server.
http://www.symantec.com/business/support/index?page=content&id=TECH95353&locale=en_US