Correct. You can run whatever you want. ESXi is the hypervisor that runs VMware. So yes, you can host many different servers(VMs) on one ESXi host. You use vSphere in order to setup and access your VMs as you would in a typical enterprise. Its great for a home lab because you can setup many different servers with different OSs and play with shyt. If you break something, you just roll back to a snapshot. I prolly got like 20 different servers on it. Though i only run 5-10 at any given time.
In my lab in addition to my domain and exchange servers, i have a few NAS sims (Netapp, EMC, Isilon), some security softwares, as well as my private cloud and media server(Plex) to stream movies and shyt.