I use a little bit of cedar in the litter mix of my mouse cages as it helps to deodorize the aquariums they're in. However, I don't use primarily cedar, just enough to lightly cover the bottom and I can barely smell it when I'm done with the other products.
I use primarily pine and aspen in my mouse colonies. I use this order: cover the bottom with World's Best Cat Litter (made from corn and super absorbant), small amount of cedar (1 handful), pine to cover the cedar, 1-2 inches of aspen, a sprinkling of care fresh in the corners. Since using this combination, it's been 2 weeks since I last cleaned them out, and no mousey odor so far =D
Anyway, with using this small amount I haven't noticed any problems so far with my mice.
As for guinea pigs I've always read and been warned to NEVER keep them on cedar. I saw it first hand as my best friend killed her first guinea pig within a week on cedar. They're extremely sensitive to the fumes and oils. I always kept mine on pine or stable shavings instead and never had a problem.