I wanted to stand up a new VM to host SSO and Web Client service, another VM for vCenter and the Inventory service.
You can keep SSO, Inventory & vCenter server role on same machine
We were running 5.0 and MSSQL on the same VM in our 5.0 environment.
If you have SQL DB cluster in your Infrastructure, then it is recommended to migrate vCenter DB to that server
Then I wanted to point that new set of resources at the instance of MSSQL running on our original VM from our 5.0 environment,
Do you want to start installation and give DB name as old vCenter DB? Not a good idea. Rather perform online upgrade (Even de-coupling of vCenter DB is possible)
vCenter 5.5 installation is bit different than 5.0
Can I do it this way? I know I have to make a change in my existing vCenter server to put it into linked mode, and then bring up the other one in linked mode as I install it
AFAIK: Not recommended and Linked mode is different concept which you can't use in your case
Can I stay with the same Database instance and have 2 vCenters pointing at it at the same time?
Bad Idea and screw the vCenter services
Using new DB for Installation and moving ESXi servers to vCenter 5.5 has only one risk of loosing performance data & Cluster configuration but works good.
Based on what your customer/policies agree to loose, you can create your UPGRADE PLAN