David Gerrold discusses in his book about the creation of that episode. He admits with some chagrin that the whole "fluffy critters who eat many times their own weight" concept had to have been inspired by "flat cats" (he's a Heinlein fan), but states that he didn't consciously rip them off. Frankly, he did such a nice job on the episode as a whole that I'm willing to give him the benefit of the doubt.
I, too, don't care, Tribbles is one of my favorite episodes!