Yes, you can use SFP-H10GB-CU3M to connect 6120XP to Cat 6k, with the help of CVR-X2-SFP10G converter for X2 ports. This converter will convert existing X2 based 10G port on Cisco 6500 to SFP 10G interface.
if we don't have this connector, what kind of cable/connector should we use to connect a UCS6100 FI to a cisco switch?
Fiber cable + SFP 10GE on the nexus side and the other end of the fiber + X2 on the 6500 side
Is it compulsory to use a Nexus equipement on the Lan side?
I have a cisco 3750 switch with 4 miniGbic ports, i can't use a UCS SFP+ from the Fiber-Interconnect to the 3750.
You dont have to connect to Nexus equipment. Your LAN can be anything as long as you can connect the UCS system to it.
Currently you need a switch with 10GE ports to uplink from your 6100s.
I believe that in the next version of the firmware/software in UCS you will be able to connect your 6100s to the LAN using 1G ports.