One issue is resolved, I figured out it was a permissions issue. I had to log in with the SSO admin account I created and add AD domain admin group with admin rights.
In my vCenter 5.5 web client, I was going to try to add the hosts anyway just to see if there was connectivity there (figuring an error message might tell me that much) but I don't see a way to manually add them. It appears when I select hosts, it starts scanning for them (and finds nothing which is what I'd expect).
I just want to make sure this appears to be behaving correctly given that there are no 5.5 host agents out there.
So the only questions that remain are:
Is the agent I need to update on each host backwards compatible with vCenter 5.0?
What is the best way to rollback the agent on the host if I encounter any issues?