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.

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

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

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

STL files available on Thangs

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