There's no reason this won't work. Even though this is a small cluster, it might make sense to set things up ahead of time with multi-site. This would allow you to expand each site and utilize search affinity if one site becomes the primary for a search head.
What you can do is setup multi-site, then configure your search head to be in "site = site0". This turns off search head site affinity, enabling the search head to search the cluster as if it was one large indexer cluster.
Your configs would look something like this for a RF of 2 and a SF of 1: