I think it can't hurt to add more links. It'd be nice to have a list of all of the legal distributors out there in one centralized location. As for the red line... I think anything notable/legit should be fine. I would say definitely include the link if the author/artist tweeted it, or if it's listed by the official publisher.
It'd also be nice to have a centralized list of links to all the manga indexing websites out there. As for which ones to include... Well, for comparision, the
/u/Roboragi anime/manga lookup bot on Reddit typically includes links to AL, KIT, MAL, A-P, and MU for manga requests.
Related discussions:
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#5101/63988: More external links e.g. Anilist, Kitsu and Anime-Planet
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#5101/66641: Add ebookjapan to links
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#5715: Links to official releases in other languages
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#10633: Custom manga external link names
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#12979: Add VNDB link