Best practices: small communication sites

It depends...

The justification of separate communication sites linked into the hub come down to the ownership of the other communication sites attached hub site. Say you have a internal communication department running the Hub site - they would post really broad (things applying to the entire company) articles and notices here. The attached communication sites could be owned by other departments with their own article set keeping their department at the focus of the articles.

At the hub level - the internal communication department then could rollup content from the attached sites (use audience targeted web parts or by recycling published content - i.e. Congrats to our sales team for making the deal! Read more about it here).

Soooooo, if you can't justify using multiple sites and you're going to use a single communication site just make sure to store your documents and pages with some sort of information architecture in mind to group the documents together. Then if you need to grow into a multiple site implementation it will be easier to identify the documents and pages that need to be moved based on potential ownership and audience.