Technically yes you can have a Cluster of a single node and if you are thinking of adding another node into the cluster later on it makes sense to set the first node up.
This both from a technical standpoint, but more importantly a BAU point of view. The promotion to mulitnode cluster will be less intrusuve as there will be no extra software to add to the pre-existing primary node, therefore you Change Board will be less nervous about the peice of work, as it will not be changing the pre-existant node.
From a personal point of view my current client is running a number of single node clusters, in their environment.