R6.5.5 to R8.5 upgrade questions

Our company has a Notes Domain that contains 6 Domino Servers spread over 4 Countries. One server is the Admin Server running the SMTP Gateway which handles all incoming and outgoing email outside the company. Another server is running the Blackberry Enterprise Server controlling all our Blackberry handhelds. We have approximately 150 Notes clients. All the Domino Servers are running R6.5.5 as well as all the Notes Clients. What is the correct order for doing a R6.5.5 upgrade to R8.5?

Can you upgrade directly from R6.5.5 to R8.5?

Are R6.5.5 clients compatible with R8.5 Domino Servers?

Are R6.5.5 Domino Servers compatible with R8.5 Domino Servers?

Would you upgrade the Admin/SMTP Gateway server 1st and then BES server and then the other servers in that order?

What are the software tools/options available for upgrading Notes Clients to R8.5?