Interesting question. What if the business is a 24-hour shop. They have one size/version of Session Hosts that users are working on and a new size that you're working on moving users over to without interrupting their work. One option is to add the new Session Hosts to the Host Pool, gradually configure drain mode on the old size/version, and remove it.
In general, having different Session Hosts in the same Host Pool is not recommended, as load balancing will either overprovision users to the smaller Hosts or under-provision, the larger hosts, depending on how you have it configured.
This is also an unsupported configuration per Microsoft.
