Cisco WAN :: MPLS MTU 6500 With SUP 720

Just like to ask first your inputs about the MTUs needed on our proposed setup. We currently have a large internal network composed of several metro ethernet links. We have different carriers and we all know that they do not always provide L1 connectivity. They sometimes do Q-in-Q or EoMPLS or other technologies that would hide their internal network and appear as a point-to-point ME circuit to customers.
 
We are planning to create our own MPLS network for our clients so we don't have to leak their networks inside ours and we are trying to avoid the overhead of GRE/IPSEC since we'll be adding a lot of client networks and the overhead is not reasonable. So we just thought of MPLS-VPN to at least reduce the overhead and we don't have to purchase a lot of network devices.
 
With that said, what is the safest thing to ask the carriers and what settings should i put in our network devices. I am still confused with the differences of MTU, IP MTU, and MPLS MTU.
 
We also have one circuit running 802.1q instead of using routed-port on the switch. Is 802.1q supported in LDP?
 
Our internal network is comprised of 6500 switches with Sup720 and Gigabit linecards and we are planning to use 3900 routers as PEs. We all hooked up our ME circuits across the 6500 switches.