Unlocking new NPCs is just giving old ones?

It seems that if you edit the base NPCs like Kobold and Crocodile to be other custom NPCs, and then you unlock new NPC slots later, the new NPCs you get will be kobolds and crocodiles. This is very annoying, I wish it would unlock new types of monsters for me to save me time so I have a foundation to start on, some inspiration as to what the game intended those slots to be if I had not customized those early NPC slots. Thanks.

You can edit the NPCs in the model editor so they have some variety.