i just have one guinea pig, but my family has
SIX cats!! Whan shes in her cage they leave her alone (all but the newest one and she keeps trying to bite him so i thank he'll lrean REALLY quick not to stick his foot in her pen 
!!). you mite whan't to be carful if you put her/he on the floor becouse they WILL try to get her/he at least once, and i find that the lazy ones are the one that will try. but i'm not saying that all cats are going to try to get her/he. my sister has a cat, thor that was a kitten when he first meat lilith so he grow up thinking she was just another kitten, he sleep ontop of her pen, brings her his cat toys trying to get her to play. some of the older one that have been knonw to kill rodents (ONLY wild rabbits and mice mind you) don't pay her any notice, even when she on the floor, so it really depends on your cats presonalltys. If i were you i would go ahead and get the guinea pig but just be careful!!