Notes/Domino 6 and 7 Forum : Domino/Lotus Notes 6.5 and license schema

OK, I have read a lot about the licensing but still not getting it. Let me explain my situation and maybe someone can help me understand? My company is a passport advantage customer and we purchase our licenses and maintenance under a CEO contract. This allows me to install 6.0.x as well 6.5. I have a couple thousand Lotus Notes users on 40 mail servers. For approx 200 of these users we have purchased a Sametime client license. These sametime users are spread out over a bunch of servers but use 2 centrally located sametime servers.

Now I have plans to upgrade 6. Do I upgrade to 6.0.x or 6.5 on my mailserver servers or not? Can I upgrade a server to 6.5 and then have my 200 sametime users work with the 6.5 client and everybody else with a 6.0.3 client? Is that legal or not? We don't have plans to purchase more Sametime licenses (definately not everybody) but I do want to give those who are licensed for sametime the integration if we are allowed. Then, do I need those websphere licenses or can I use the 6.5 server and client and get away without have to purchase any websphere licenses? Where does that websphere functionality really come in play?