generally guinea pigs don't have a problem with chewing fleece. My piggies literally have no interest in chewing it at all. I wish I could say that about my plastic litter box I have for their kitchen though! >:[ haha, I wouldn't worry at all. I've really never heard about that happening, and if that actually were to occur, you'd be able to catch it soon enough and then switch back.
Litter Box Training a Guinea Pig | ThriftyFun
Both female and male guinea pigs can be territorial and like to scent mark around the cage. Often guinea pigs will choose corners in their cage to go to the bathroom in, constantly returning to the same corner. When you have seen that your guinea pig has chosen a corner, place the litter box in the spot. Put your normal bedding in the litter box before trying any new types of litter.
One way to save money on guinea pig supplies is to use a litter box
If you want your pigs to only use their litter box and never have accidents when they are out of their cage, then perhaps you are setting your standards too high. Be realistic. If your guinea pig manages to use it’s litter box even just 50% of the time, it will still help keep the cage tidy and reduce your work for cage clean outs.
Bedding Bedding and litter box use - Cavy Cages
The trick to litter box training guinea pigs is patience and consistency, as they generally do not take to using litter boxes immediately, as cats do. And, guinea pigs poop … a LOT. This doesn’t mean that all hope for a house-trained pig is lost, though, since they naturally recognize certain areas of their living space as ‘bathroom’ areas, and others as ‘clean’ areas. A guinea pig with adequate space in their living environment (at least 4 cubic feet of space per pig) will not eliminate randomly around the area, but should instead eliminate in places of their preference, primarily in quiet corners of their home.We recommend lining a litter box with a few sheets of newspaper, to make clean up easier, then placing a layer of litter one to two inches thick on top of the newspaper. Recycled newspaper-based litters, like Yesterday’s News brand litter, are a suitable substrate because they are not toxic, dust-free, and fragrance-free. Maybe typical cat litters are not suitable for guinea pigs, so make sure that you speak to your vet, if you are unsure. On top of the litter layer, there should be a pile of hay on top to encourage your pig to use the box and to serve as a reward for using the box. Top each litter box with a few fecal pellets as a reminder of what the box is for.