How can I monitor servers and switches behind a firewall of another company with my NPM server?

I am doing a similar thing.  To my knoledge you really have 2 options, first is to do a site to site VPN, the second is to have enough external IP addresses at the client site (one for each monitored device) and apply NAT.  Keep in mind that only SNMP v3 has security and that any none tunnelled traffic will not be encrypted if you use the 2nd method and SNMP v2 or lower.

One other option would be to become a Solarwinds reseller, sell each company their own Orion installation, install the EOC at your company, create site to site VPN tunnels between your site and your customers' sites and monitor all of the sites via the EOC.  Ideally, you would charge each customer a monthly monitoring fee for this service.  And yes, I used to be a consultant!:)