Personally I wouldn't do any network discovery through Prime Infrastructure as Prime is not really a network monitoring tool but is a network device management tool. I would strictly import devices to be managed via the import csv option.
Licenses are very expensive and you should know what's out there on your network and at what level it needs to be managed.
Not only do you need correct RW SNMP settings to make full use of the tool but you also need login credential to ssh or telnet into each device.
I would go out and use a freeware tool like Network View or Alloy Discovery to see what's on your network then go knocking on the folk's doors who own those pieces of equipment and ask them to let you know how it needs to be managed.
Prime Infrastructure is a costly and very complex tool that will require a lot of your time as it is.
Try to use it for meanial tasks you could use other tools for and you'll find yourself spending way more time than needed to keep it running.
Just my two cents
