1) How from vCenter can you see which hosts are ESX and which are ESXi. Can a single report churn out which hosts are running what? Could you provide a screenshot on how to get to this?
2) Can you have a mix of ESx/ESXi hosts in your environment, if so does that cause issues?
3) Which came first, i.e. is ESXi a newer version of ESX, or vice versa?