Don't quote me on this .. but I think the HCL list contains "tested" devices.
Neither of the NICs I listed are on the list for XS 6.0 but they work fine.
I believe it is more important to pay attention to the controllers used in a certain NIC
and verify a driver packaged with XS can work with your NIC.
(ie ... all my Gbit Intel NICs should all be covered by the included igb driver)
I didn't see Realtek 100Mbit NICs on the XS 5.0 list, but they worked fine in my very first XenServer lab.
Going forward though ... I am sticking to Intel and Broadcom NICs.