Yeah you have to be careful with mulch--cedar mulch is absolutely toxic, and a lot of mulch is polluted because it's stored badly, surrounded by car exhaust, etc. I use cypress mulch only, absolutely no dyes, sanitized by baking it, or the expensive pet store mulch. But cane toads pee so much that I doubt you want to keep bying pet store mulch. I always feed outside the enclosure, but if you feed inside the enclosure and your toads will take moist prey like earthworms, I would feed the worms outside the cage to avoid ingesting mulch.

I wasn't sure what you meant by the guinea pigs, but I wouldn't put them together w/ cane toads even for a few minutes--the cane toad venom can blind or kill a curious guinea pig, and a curious guinea pig with teeth almost an inch long could take a quick chunk out of a cane toad. I raised guniea pigs for 10 years as a kid, and they look placid but they think everything is a snack. It only takes a split second for something really bad to happen.

Cane toads are also pretty dirty, especially if you use a soil mixture in the substrate, so that's why the hand washing is important as much as for the venom.