Mechanicsburg is heavily implied to have a fully functional Mouseworld going on. The mice in the Red Cathedral especially (they have knives) but also the rats (the boy who digs up a dead rat assures Krosp, in the novel, that he was an out of towner and no one will ask questions). The birds are sometimes seen reading maps and smoking cigars.
We don’t see any mice in the Castle and I don’t blame them, what with the venemous rafter toads, etc, although the Castle claims “ghostmaker mice” could have infected Tarvek with Chromatic Death. No word on whether those are smart mice.
The nyar-spiders (Mechanicsburg Variegated Knife and Fork Spiders) are ambiguously smart. They do use cutlery.
The mimmoth armour might just mean someone decided to armour mimmoths at random, but might also imply mice riding into battle alongside Mechanicsburg’s other residents at least once.
(I don’t get the impression that the humans in Mechanicsburg never kill mice so much as that… well, the monsters in the tunnels kill humans sometimes if you intrude into the wrong place at the wrong time. Just be smart around the species bigger than you and you’ll be fine.)
Then you’ve got to take into account that Agatha’s recently introduced the dingbots who are like a tiny, Heterodyne-devoted kingdom of their own. But they’re mouse-sized. If there are mouse-houses, let alone mouse-workshops, under Mechanicsburg’s floorboards they’re bound to stumble on them.
The first test of whether a large society of clanks can live peacefully alongside more organic constructs is possibly going to be Smol.