The lamia were living in a dungeon though, so they probably didn't do a lot of interacting with other races. The centaur were in a village that got caught up in the war with one of the human kingdoms, I think. Maybe they were trading with them before the war?
Regarding the ranking system. I think if the only metric they measure rank on is power, it makes sense for people to jump ahead. If being high rank means you get jobs where you're responsible for guarding royalty, I would expect them to require the adventurers to build up a history to show...
You can't make the medicines he was making with herbs, though? It requires a level of infrastructure and knowledge of chemistry that doesn't exist if they haven't even figured out microscopes yet.
The guns are deadly, the shotgun is supposed to be firing rubber though. Despite how she is portrayed, she doesn't actually find the idea of killing people exciting.