Slitherine

In the Lowlands was purchasing some extra core units. Showed I had 22 slots after buying 3 units and still had plenty of money to purchase it showed that I had 0 core units slots left. Are Core units 1 slot or do different units have different amounts of slot allocated to them?

That is something I would like to figure out too. How do I change core units? Delete them as you could in PG?