It has nothing to do with aggression or predation, it's just that Mbu's are curious and the primary way they "investigate" things is to sample them with their teeth... And you can imagine how that could turn out for a basically stationary fish like a pleco.
The fish that have done best with my Mbu are ones that are small and know to "get out of the way", such as Tanganyikan cihlids. For dither fish, we have a school of about 30 celebes rainbows in there that add some action to the middle/upper water column. I'm sure your guppies will be fine, too.
I'm going to agree that there is always a definite chance a puffer will attack any tank mates its in with, really depends on that puffers personality and what they want to do...