The ports on the Nexus switch are 40G/100G. The port on the Catalyst switch is 10GE.
You will need to use the CVR-QSFP-SFP10G adapter or a 4x10G QSFP breakout on the Nexus switch.
Then you just need to match up the 10GBASE standards on both ends.
The 4x10 breakout gives you single mode fiber, copper, and AOC options.
The adapter should let you do anything--copper, multimode, single mode, AOC.
You will be limited by the pluggable choices on the 6506. For X2 and XENPAK, you will only be able to do single mode and multimode fiber. If you're using one of the 10GE ports on the supervisor card, that's X2, but there's also an SFP adapter available.
This link should help:
https://tmgmatrix.cisco.com/
