I have two servers that need to use v6 to access a second nic on the box, but my vSphere Server license is v5.5. Is it possible to manage those two hosts at v6.0 with a v5.5 vCenter Server Appliance. Alternately, Can I use a v6.0 vCenter Server Appliance to manage the v5.5 vSphere host?
Newer vCenter versions can usually manage older hosts, but not the other way around.
vCenter Server 6.0 can manage hosts with versions 5.0 and later (see VMware Product Interoperability Matrixes)
André
And just be careful with licensing here. If you do install a later version of vCenter to manage the 5.5, you will have to use the proper license for it (should be different than the key for vCenter 5.5). That should not invalidate the licenses for the hosts themselves.
You cannot manage ESXi 6 hosts with a version 5 vCenter in a supported manner. You must use a higher version of vCenter (so 6) to support ESXi 5.5 and 6 hosts. As people have mentioned you will need to update your license keys