Toy Rotation and Specific Toy Question

[mufasa]

I've read in a couple places that it's a good idea to rotate your piggy's toys so he doesn't get bored. Makes sense, but how often? Is once a week good? Every few days? Ive owned Mufasa a week, and I just did his third rotation today, but I suspect I'm overdoing it. Of course, since he's new here, I'm also trying to get a handle on which toys he likes best.

That brings me to the specific toy question. I've also read that hard plastic cat balls are okay for guinea pigs. Is that true? Even if they're solid, hard plastic, I'm worried he could somehow chew them. He loves to push things around his cage, and I have some very solid balls (the Angry Birds cat toys with a bell inside), but I don't want to give him anything that would potentially harm him.