"Less cats" and "fewer cats" both mean "a smaller quantity of cats" if that's what the speaker intends it to mean.

DOWN WITH PRESCRIPTIVE GRAMMAR! LONG LIVE DESCRIPTIVISTS! *waves banners*

I heartily endorse this comment. Language belongs to the speakers, not to some higher authority. As a writer you should probably learn ways to express yourself that aren't going to muddle up what you're trying to write (and that's a different story all together), but "proper grammar"? The only "proper grammar" is the grammar that lets you communicate with others.

VIVA DESCRIPTORAS!