There are other enemies in the world which are immune to bleeding. There are also other enemies in the world which fight each other, look at exiles and the crocs at noob river.
As for the other point, this might be considered a suggestion to re-allocate them.
In this case, according to lore, they are supposed to be friendly (inside the dungeon they don’t attack each other).
As far I’m aware immune to bleeding are supposed to be undead. Serpentmen Brute is a living creature. I think the problem here is that for some reason is using their undead version template.
Thanks for the report. We’ll inform our team about the lore conflict. Regarding the placement of those enemies, we’re aware of it was it was reported a while back and it’s on our team’s to-do list.