I don't necessarily agree about Croaker, since he's more passive early on, but the guys doing the action, like Elmo, Raven & the three wizards are going to be more important than a long view of the series might suggest. Silent might be the most important casting choice of the magic trio in the beginning as well, since he doesn't have the dialog or an easily quantified relationship with another character to establish himself, but One-Eye & Goblin are going to be there for the long haul.

But I wonder if they might make Lady much more involved in the early going to get the whole "strong female lead" thing, which could definitely spoil her character.

I agree on the purist position, and IMO, one of the good things about "The Black Company" is how sparse it is, and how much leeway you have. It's nothing like aSoI&F where you can look at the books and look at the show and say "Wow, that's different," leading to either, "Why are they doing this?" or "Well, it's obvious how they're pandering here!" They don't need to break as many parts to fit it into a TV format as they do for GoT or, maybe someday, WoT, but if Lady is their priority in casting, that might indicate they're tampering with it already. I think Lady works best in the beginning as the mysterious, remote figure of awe and terror. A lot of her appeal later on is informed by that original presentation, and she engages the audience or reader by stepping down from her pedestal to interact with Croaker. Their relationship is flawed, but they make it work, because he's the one individual who cared about her as a person, even if he had no basis on which to build any opinion about said person, and because of the degree to which she reached out to him. If she takes over Soulcatcher's role and is more of a hands-on supervisor to the Company, that messes with the dynamic, and makes her too familiar.