Oh no, you’re not doing anything wrong. What I illustrated earlier was not ‘right’, either! Those are preferences I have for character design, but more catered to toonish stuff, specifically for a project I’ve been working on that utilizes that style. Shapes don’t have to be as cartoonishly exaggerated as in the previous example, so much as have life in them. This is as simple as displaying casual body language between your characters. The whole silhouette tip should be considered a loose guideline, or a way to warm-up or brainstorm, but not an absolute method for character design. There is no one method, that’s the beauty of it. Your method will differ from mine, and as a result, your characters will look like just that, your characters. That matters just as much as making them distinct.
Again, a lot of it breaks down to personality. Give your lines personality, and movement that helps indicate it.
Here I used a few folks with similar character features. Ree and her classmates. Uniforms, age range and even heights, all roughly the same. No giant hunchbacks or spiky protrusions to distinguish them, but you can still fall back on body language to help depict their personality. Even similar outfits can be drawn differently from character to character to help. Remember, I’m no expert! Anything I say here should, at most, be taken as a suggestion or a something-to-try-out. :] Thanks for writing!