STL files available on Thangs

Yes any standard sized utility knife blades should fit. I tested a bunch of different brands from my local hardware store.

I have the STL files on Thangs for 3D printing.

My two are pretty good with not eating things they shouldn't so I just made sure stringy bits were cleaned up. Googling says PVA/wood glue is safer for cats but I would check brands if they are non-toxic

Yes they can easily be scaled to fit them. I have 73mm and 85mm holders. The smaller one is pretty universal for most store bought bottles and the bigger one is more for cups and glasses.

I have a 3D printer buyer's guide on my website that lists a few. I mostly use and would recommend any Bambu printer, there's a few that can suit any price range. Elegoo also make good printers too which won't break the bank