Hi I have seen customers with upto 50 hosts using the SSO on the same server without a glitch. I highly recommend you see this and go for an inplace upgrade after backing it up. You can either take the back up of the VC 5.0 or
back up the DB of the VC 5
or
take a snapshot of the VC 5.0 before an inplace upgrade of the VC 5 to 5.1. It will be eaiser and quicker than you think but still i suggest you plan your downtime & do it.
http://defaultreasoning.com/2012/09/12/upgrade-vcenter-server-5-x-to-vcenter-server-5-1/