I've never heard of Unbeholden's dark monsters addon. However, there is a vanilla version of this mod, which is the most multi-compatible version (as the Guncaster version is... well, specifically for Guncaster). I have no idea if it's compatible, but you're welcome to see if it is.
On the other hand, I do know of Project Brutality. A mod dear to my heart because of its awesome weapons and effects... but most of all because of its bestiary. I was also making a Brutal Guncaster patch, and once I was done with that, I would make a Brutal Guncaster version of this mod, and get all the brutal monsters I can get my hands on.
Ok, that's besides the point... if you would like to implement these monsters into Project Brutality, then be my guest. I always love to see Brutal Doom versions of custom monsters.
Just be sure you give credit where it's due.