Hello Nael. It is supported to have the VSM (as a VM) to reside on an ESX/ESXi server that is also a VEM. The thing to make sure is when you migrate the VSM interfaces (control, management and packet) behind the VEM, the port-profile for the control interface must be configured with the "system vlan" entry. Here is an example where I have the control and packet interfaces on the same vlan, which is VLAN 50:
J05-VSM-01# sho running-config port-profile n1kv-control-packet
!Command: show running-config port-profile n1kv-control-packet
!Time: Fri Jan 27 21:50:08 2012
version 4.2(1)SV1(4a)
port-profile type vethernet n1kv-control-packet
vmware port-group
switchport mode access
switchport access vlan 50
no shutdown
system vlan 50
state enabled
Note that I have the entry "system vlan 50". This way when you migrate those interfaces behind the VEM, the communication between the VSM and VEM is brought up properly. This is explained in the Nexus 1000V Deployment Guide.
As to your question about clusters and such, customers typically have a management cluster to host the VSMs and vCenter. It is not required but does defined a couple of servers to management functions. If you are using VMWare's DRS, just make sure that you set affinity for the VSM VMs. This way you want to make sure that both the Primary and Secondary VSMs are not on the same physical servers. Other than that, you should be alright.
