Howdy and Welcome.
I think you'll find the biggest issues with used tires < tyres> will be the dirty factor and the Poky Steel factor. Used tires are extremely likely to transfer grit, grime and black stuff onto everything they come in contact with. A thorough cleaning with a
wire brush and soapy water will help alleviate a lot of this, using paint to the put the dirt back on them will help even more as it will seal the exterior of the tire. < tyre>
The Poky steel issue is one you have to confront at the time you procure the tires, if the tread is sufficiently worn to a
point that the steel cables on the interior are poking out don't use 'em, unless of course they are somewhere way far away from human activity. You will of course need to make sure your carps < chippeys
> are informed of the potential poke hazards. Puncture wounds are the worst; they hurt the most, they have much more potential to become infected, and they hurt!
If there is any chance of the tires being exposed to and open flame or any other combustion source, then my choice would be to use something else. When exposed to open flame tires can catch fire extremely quickly and once burning they are intensely hot and extremely difficult to extinguish, That why you see them burning in the streets at a riot, it's easier to just let them burn out than try to extinguish them.
Good luck.