Well thank you both for this. The classes being referenced elsewhere was the problem I got after the simple renames. Because of that, and Badger using Instigators as an example, I decided to poke the Devourer with a stick (as Instigators aren't out yet).
So far he's renamed the Cookie Monster, has his own name, description, faction description, game entity and unit tags, as well as his own named long term planning, though it's just the same as the original. Been trying to replace it with that of an Enraged Macrophage Harvester, but it doesn't want to be angry. Think on that front I'll wait for Instigators, as that's a pretty simple code from what you're saying, so it should fit in much more easily.
Also figured out how to make it not spawn anything at game start, too, in the C code.
The only other oddities I encountered was, I tried to make the Monsters XML data as a mod to be loaded, but it never did. I've been able to get the game to load in things very similar to the example mod (a Thanatos speed change to "Blitzer"), but adding in a new SpecialFaction to the list didn't work, so I just threw him in the...main? file, and he's happy enough there. I tried having an XML of just him put into that folder, and a copied version of VanillaEntries with him added in, but neither worked. No errors, he just never showed up.
Second, I tried to give him the Fighters gun...but he very stubbornly hugged his DevourerGun close. I imagine this is a moot problem though, since this is changing in the Era of Discovery update. Still, curious.