Re: EX4200 with CAT6 cabling

It should work.

Cable Specification for 10GBASE-T Network Ports and 10GBASE-T Expansion Module Ports
Ports on Switches IEEE Cable Standard Cable Standard Specification Maximum Reach Frequency (MHz) Additional Information
10GBASE-T network ports and 10GBASE-T expansion module ports Class E/ CAT 6 Unshielded Twisted Pair (UTP) ISO/IEC TR 24750 / TIA TSB-155-A 180 ft (55 m) 1–250 CAT 6 cables are defined to perform at frequencies up to 250 MHz and are sensitive to alien crosstalk. To use such cables to connect to 10GBASE-T ports, you must ensure that the cable installation is tested and is certified for 500 MHz as per TIA TSB-155 standard.
Class E/ CAT 6 Shielded Twisted Pair (STP) ISO/IEC TR 24750 / TIA TSB-155-A 328 ft (100 m) 1–250
Class EA/ CAT 6A ISO/IEC 11801:2002 Amendment 1 / ANSI TIA-568-C.2 328 ft (100 m) 1–500
Class F/ CAT 7 ISO/IEC TR24750 328 ft (100 m) 1–600
Class FA/ CAT 7A ISO/IEC 11801:2002 Amendment 1 328 ft (100 m) 1–1000

From the above table we could find that the EX4550  A 10GBASE-T can be converted as a VC port and also supported the CAT 7 cable with the above specification on the interfaces.

Reference link is below:
http://www.juniper.net/documentation/en_US/release-independent/junos/inf ormation-products/pathway-pages/ex-series/ex4550/ex4550.html
http://www.juniper.net/documentation/en_US/release-independent/junos/top ics/reference/specifications/cable-ex4550-network.html