If the service you are interested in on the "customer's" server is available through snmp, then no, you don't need to install anything. Also, the ftp checks, http checks, sql checks, and many others, are not invasive. They can be configured to actually login to an ftp server, though, which is a darn good sign that the ftp server is running. Is that invasive? Perhaps to some, yes, but how else are you going to know if it's up?