'Ello! Mun for small-lost-rat! I'm curious as to your headcanons on Rats.
Henlo! :D
Okay, so, my headcanons will mostly be a sort of "addition" to yours because I like yours a lot and I'm realizing that mine are similar to them.
Anyway, let's see if I can put my thoughts into words:
I like your idea of the Rats having a polytheistic religion, also because this builds on what we suppose is canon (at least because it's written in the wikia), the part where the masks have become a sort of "sign of status" and used in "ceremonies and sacrifices".
Occasional cannibals like most Pandorans, or just a little more often depending from group/clan to the other esp basing on what caves or ruins are inhabiting. Like...if in that area is more difficult to find a source of food, they'll have to resort to cannibalism more often obviously.
Another part of your headcanons I like is the "created in a lab to find Eridium". That fits so much with the "Lab Rats" that you could find around, more in the area of the mines near Lynchwood - IIRC - so this could be a perfect explanation! And also could fit well the fact that they have claws!
I also headcanon that the first ones were people that have mutated after being exposed to/in contact with Eridium and other dangerous minerals and materials for too long. An example could be what happened at the Headstone Mine back in BL1, where the miners and workers there started "going crazy" and mutating becoming the first Psychos. Some could have become Psychos, other - maybe depending on genetics or maybe how much exposure they had - could have turned into Rats. (this could also go well together with your headcanon about them having a penchant for stone/lapidary)
I agree with you about them being subsistence thieves: in caves, old mines and other places there isn't much and after a bit, you'll have to go find and steal what you need to keep surviving. Some might have taken a shine on the looting and tend to do it more often and keep also things not needed for survival but because they like them, but most of the Rats steal and loot only because they need things to keep living. As any other Pandoran going around and looting hoping to find anything useful to survive.
Aaaand that's it, I think. Honestly I never completely fleshed out my HCs on the Rats so probably it's not much lol