Dear,
You can use the vCenter 5.0 or 5.1 with ESXi 5.5?
The vCenter Server version must be always higher (or equal) that the vSphere ESXi version.
You can check all interoperability matrixes here: VMware Product Interoperability Matrixes
You typically want to keep your vcenter version => than your esxi version. I can't say for sure that you cannot but why would you?
The vCenter Server version must be always higher (or equal) that the vSphere ESXi version.
You can check all interoperability matrixes here: VMware Product Interoperability Matrixes