As you have found from the documentation, moving from the 1.5 train to the newest code is fully supported from a VSM perspective. What's not documented is the little bit about the VEMs. In general, the VSM is backwards compatible with older VEMs (to a degree, the degree of which is not published). Although it's not documented (AFAIK), the verbal understanding is that the VEM can be a version or two behind, but you should try to minimize the duration that you run in such a configuration.
If you plan to run mixed VEM versions while getting your hosts upgraded (totally fine that's how I do mine), it's best to move to that upgraded VEM version at the same time that you upgrade the hypervisor. Since you are going from ESXi 5.0 to 5.5, you will create an ISO that contains the Cisco VIBs, your favorite async driver (if any), and the ESXi 5.5 image all bundled together so the upgrade for a given host is done all in one shot. You probably already have that technique down cold, but the links generated by the Cisco tool below show you how to do this. It also gives some handy URLs to share with each person performing duties on this upgrade. Here's the link:
Nexus 1000V and ESX Upgrade Utility
PS - the new thing is taking offline clones of your VSMs. Although they are easy enough to recover, having an actual pure clone will save some of the secret sauce you might otherwise lose in a failure scenario. Simply power off one VSM, then right-click and clone. Power that VSM up again and failover the HA pair, then take down the other one and get a clone of it. So as a safety measure for this upgrade, get your offline clones of the current 1.5 VSMs, then sometime after your successful upgrade get some offline clones saved of the new version.
