The suggestion that they should all be acquainted from before the series does not really hold.
I’ve not suggested that, you have.
What I have said is that if you have a chance to iron out a bump in the continuity, and it doesn’t affect the story you are telling, it’s expedient to remove it. I’m not just making this up, this is how scripting an adaptation like this actually works in the real world.
Calculus’s introduction in Red Rackham
as first an irritant to Tintin and Haddock, then a stow-away was obviously an appealing enough plot-point to render his introduction worth keeping. He’s also in enough stories that he is someone with whom a viewer will quickly become familiar, not just a figure who pops in and out of a couple of episodes, causing head-scratching (“Have we seen him before? Where? When?”).
Skut on the other hand is a very minor character (albeit a popular one with some fans), and in bringing him to the screen losing his entry-point to the cycle makes sense, as it covers far more bases: you can watch the stories in any order; the viewer is free to think that the episode they first see him in is his first appearance, if they so wish, and it removes any requirement to actually know that you have seen him before if you have forgotten by the time you’ve actually seen both episodes.
As Balthazar points out, he does have a bit of a “character-arc” in his appearances in the books, and there is nothing to say that it couldn’t have been approached differently in the adaptation; however, that’s not to say it would have been handled better, or improved things: it might have ended up rushed, or too cramped to be effective, or if emphasised, it might have spoiled some other aspect of pacing or structure.
Many films and TV shows (both original and adaptations) lose fantastic scenes and great performances because when looked at dispassionately they don’t fit into the picture as a whole.
It happens to anyone and everyone: have a look at Olivier’s film of Hamlet
some time, and wait for Rozencrantz and Guildenstern to arrive - because they won’t…
However Tom Stoppard thought them so important that he wrote a play which expands their part to be the focus of the action, with the rest of Hamlet
taking place in the back-ground…