I don't recall exactly now, but I thought McCoy had ran some tests on the blood prior to using it on the tribble. Testing with the tribble was simply another experiment..
A rat would make more sense. Tribbles are completely alien earth. The tribble was simply for fans. And I guess now they've encountered the trader who was selling them. Because in TOS, they never saw them before DS K7.
A rat? A rat?! More sense? This is "Star Trek" man! Get a grip! Besides, seeing the tribble, even in a cameo, made the movie for my wife. Tribbles and TVH, that's Trek for her. (BTW, that was one big tribble. And as fake as it looked, just sitting there and throbbing, I thought that had to be an homage to the clockwork tibbles in TTWT. CGI can't create a better tribble than that? I'd expect them to be able to make one do the hampster dance if they wanted to -- if anyone remembers that.)
