It is not plausible, within the conceits of the Star Trek Universe, to infer that the general nature of a species of seven billion individuals is represented by the actions of a single known starship crew.
There have, in fact, been numerous examples of starship crews in the Star Trek Universe whose behavior deviates from the norm for their species. The Ferengi encountered in "The Last Outpost" were much more violent and hostile than most Ferengi, for instance. The Federation crew of the U.S.S. Equinox certainly weren't representative of what most Federates are like. The crew of the I.K.S. Rotarran were nothing like most Klingon crews before Worf and Martok whipped them into shape. Etc., etc., etc.
So the idea that it's reasonable, within the rules of the Star Trek Universe, to infer what the Husnock are like based on one ship's behavior is, again, just absurd.
It's an infinite universe. There are infinite numbers of ways a species can act. Many times we've already seen examples of a species being absolutely unified in purpose.
No, we haven't. ST is generally very consistent in depicting alien cultures as possessing multiple competing factions, or people who deviate from their cultural norms.
Why can the Borg, for example, be so single-minded, but the Husnock can't?
The Borg are not a species; it's an AI gone mad, that's all.
Kevin was just trying to frighten the Enterprise crew away. Assuming the Husnock did send an entire fleet to destroy the colony, it would have been overkill in the extreme for Kevin to do the same. He wasn't trying to *destroy* the Enterprise, just scare them off.Seems odd that he wouldn't have "sent" a bigger fleet if he'd encountered a bigger fleet.
Fleets tend to be scarier than single ships.