You can create the cluster, and move in and out the host without impact!
Moving "in" for sure is not a problem! Moving "out"...hmmm i have mixed feeling about!
If you have a single Host in a Cluster you will loose the VM autostart feature. In the past i only use this single Host Cluster setup when i have a need for EVC ( now there is EVC on VM level as well) because i need to be prepared to add host (newer or older once) later.
Impact during working hours = 0
... maybe you can argue if it makes sense to do it with a single host ... but that's another question.