Well, I think I have a guess as to where they got the name "Cantus" from. The "Cantus Firmus" is the principle voice in Gregorian chants. In some dictionaries, "Cantus" means "the principle voice in song".
I'm not sure about the other minstrels, though. Terry Angus performed Brio the Minstrel (the Fraggle with the large Bunsen Honeydew-type glasses). Sometimes he posts here, and he might know if any of the minstrels were specifically based on anyone.
Maybe...and I'm just guessing here, Jim Henson might have thought of how he might look if he were a Fraggle, and that's why Cantus has a beard. Also, during the Fraggle Rock years, Jim Henson's hair had kind of these "tufts" just above his ears. Check out the "Down at Fraggle Rock Behind the Scenes" documentary, and you'll see what I mean. Here's another photo that might help:
http://www.muppetcentral.com/_images/performers/henson_mug.jpg
Cantus also has "tufts" of hair (or almost feathers) which are just above his ears. Cantus is also taller than almost all the Fraggles (Jim was 6 foot 3). Also, Jane Henson mentioned that Jim Henson thought like a musician, even though he was more of an artist. I'm not sure if this played any influence on Cantus, but it might be something to think about.
Convincing John