erika_(artist) vs. erika_kyoukousha

Two entries for the same artist:
http://danbooru.donmai.us/artist/show?name=erika_%28artist%29
http://danbooru.donmai.us/artist/show?name=erika_kyoukousha

The second one has only two posts, but the full name. So I guess we should be using that one?

Correct, and done, especially since reducing ambiguity and the number of tags with qualifiers is always a worthy goal. Note that I'm assuming the full name is correct, but if it wasn't, it needs to be fixed. I couldn't find it on the site but I didn't look too hard.

How about masahiro_shibata, since it's him. I don't know if there's any rule on using artist names.

http://www.animenewsnetwork.com/encyclopedia/people.php?id=7354

That definately is him, if you look at the website linked to from that bio entry via the internet archive. (c.f. http://web.archive.org/web/20070418145402/www.linkclub.or.jp/~shakan/asuka-000426.html) I think we prioritize real-life names over aliases, so another mass edit is probably in order.

Again correct and again done, though it's shibata_masahiro (柴田昌弘).