When the spawning of Tribbles got faster and faster and faster, the speed of the other Tribbles falling into place when I removed them did not quite match up to the rate of spawning, perhaps both speeds could be set equally for a fairer play? In addition, I felt that the starting level's spawning rate was very slow, and the action or excitement of the game only built up in the way later stages where I can finally feel that I'm at risk of losing (in Classic mode), but it's alright if you kept the Classic mode mechanism as it is.
What I didn't like was that, in the 'Clear The Level' mode, the additional Tribbles seem to spawn out of nowhere. I was expecting to be careful about the order that I removed the Tribbles in order to clear the levels, but having random Tribbles spawning out of nowhere removed that sense of caution.
Overall, the game was quite fun, good job! I liked the cutesy feel of the game too!