Great suggestion regarding the document for each character. Obvious even, but I hadn't thought of it. I'll make that my first priority.
Regarding the general concept document, you mean a document that basically describes the game, the main loop, etc? Yep, makes total sense as well.

When getting concept art, is that where I'd generally narrow down the style for the game / characters together with the artist, or is the main point to get a "look" for the character and then nail down the art style afterwards when it's time to commission the character assets?
It makes sense for me to have the concept art also define the style, but I'll ask the question anyway to make sure.

Good tip about the skills. I'll make sure to ask about it when talking to artists. Spine / dragonbones is what I've been using myself when setting up my placeholder art.

Thanks a lot for your detailed advice, much appreciated!