You can combine hard and soft lines fine. It's not an obstacle. The problem with pic related is misapplying them. Soft is transitions, where as hard is for edges. And even that isn't a hard rule and can be stretched to get inventive results.
>Lines as an artistic technique are meant to abstract and simplify the point at which one form cuts out of another, no outline really exists in reality,
This wording comes from James Gurney if I remember correctly, who's otherwise very much worth engaging with, but here is talking nonsense. Not because the statement is technically wrong (indeed, contours exist only in perception), but because it is irrelevant to the creative process. Edges are a vital component to visual narration, and appear all over the place. Richard Schmid in Alla Prima gives more useful insight in lines, edges, and shapes.
Not to mention this assumes physical realism as desirable, which is controversial to say the least.
>Also, I would personally recommend reducing the width of your lines if you want to go for a anime look.
Moé, like any cartoon, is flexible. Line thickness is neither here nor there, as your own included image proves. It's an aesthetic judgement, best made by the artist individually.
>In short, use your fingertips for tiny lines, your wrists for medium-scale lines and your whole arm for large, canvas-sized lines. I don't have any further resources on hand, but you can probably find more stuff on this through searching for it online.
Using your entire arm is common advice for beginning students, so they learn to maximize the physical canvas and overcome meekness but I've never heard of this gradation though and on the face of it am hostile to it. Where did you get this?
It doesn't really matter here, though as in digital you only need your wrist and at most your upper arm.
Clean lines don't exist. Only straight and curved.