The reason Brool failed as a warrior is that he simply doesn't have it in him to want to harm others, even if they are from enemy tribes. He broke his leg while fleeing, and when his pursuers saw what had happened they just left him there, figuring that nature would take its course. At the time Brool was determined that if he was going to die he would take one of them with him. (Of course, he wouldn't have been able to.)
Brool's unlearning a lot now. It's tough, realizing that some of the things that you've taken as basic, unshakable facts of life are terribly wrong. Fortunately he's quite flexible for one of his kind. Mental adaptation is unusual in the subterranean world, where people live in tight little colonies and think other people are weird if their cultures are radically different.
Cantus is gifted both with musical talent that could be considered magical, and a flexible mind, and furthermore he has the desire to explore and learn. To be a Minstrel, one must have those qualities in some degree, and music may not be the primary qualification. When Cantus tested Mokey, when he set tasks for Gobo and Red, when he challenged Junior Gorg, he didn't test their musical ability, but their inner qualities, and in Gobo's case he taught lessons that Gobo needed to learn right then. It's amazing how Cantus tends to be right where he's needed, innit?
Of course, at this point in the story Cantus is still being shaped. He's learned the hard way that preaching isn't going to get his point across, not with Fraggles. To quote a famous lowercase-N nanny, "a spoonful of sugar helps the medicine go down." An I think that having descendants* will make the future more real to him. He wants to change the world for the better; now he has someone to change the world for.
Cantus singing LittleUnnamedOrangeFragling a lullaby? That's fan art just waiting to happen, it is.
I'd considered showing Clio's birth here, but decided against it because her colony is close to Fraggle Rock, and he doesn't go there until later in the story. Maybe we'll revisit that place later.
* Yes, plural. Eventually very plural. Cantus is a popular guy.