[David Cogman]

There is a lot of exposition on speech in the Pali Canon.  Try Majjhima Nikaya 58 as a starting point.  That discusses more from the perspective of the Buddha’s speech, but he’s clearly giving advice to be used by a layman.  However generally speaking, I believe the distinction you highlight is between speech which contains no merit and therefore is a waste of time and a distraction from practice, and speech which is actively harmful to others, which are two different types of fault.

On the ‘divisive’ point, it’s worth remembering that there are particularly serious consequences if this speech concerns the Sangha. The fifth of the Five Heinous Crimes is creating a schism in the Sangha. These five acts (the other four are killing one’s father, mother, or an arahant, or spilling the blood of a taghagata) result in rebirth in a hell dimension in the next life. Also speech of this kind can be the cause of two of the fourteen tantric root downfalls, which if left uncured for enough time result in complete loss of one’s vows. So saying the wrong things about one’s teacher, fellow students or Sangha is a very, very big deal, something people often forget.