This is exactly the type of mod I am looking for and it is awesome.
I am using the 1.12.2 version of the mod.
With the latest forge for that version (14.23.5.2847).

I am wondering if there is a way to add an option in order to spawn custom mobs specified in the config file.
I noticed that in this line line you are getting a random monster in the dungeonhooks that are registered. But what about mobs not on that list or are passive?

For example I would like to spawn a couple of Sirens from Ice and Fire plus/or 3-4 silverfish. (ID = iceandfire:siren)

Thank you very much for your hard work!

Added in v1.3b1 for 1.16. Will be backported for 1.15 eventually, but not for 1.12.