Regarding the identifiers, a few things to note:
Homepages
Homepages can change, as an artist might get more popular, and decide to have their own domain and dedicated site, versus using free hosting. Some might switch which blogs they use (for example when Pixiv opened up their blog service with Livedoor)
Circles
Circles as identifiers are something to be careful about. It often boils down to these edge cases:
The artist has a main circle, is the sole contributor, and they do all work through it. Likelyhood of this changing is very slim.
The artist is part of a collaborative circle. If the circle breaks up due to people getting jobs, or things just not working out, it might cause confusion for people who aren't familiar with how the circle system works.
The artist is guest for a circle's collaborative work of some kind. This is the one to watch out for, as it improperly identifies the artist in general. Generally the person who coordinates the collaborative work uses their circle name, which everyone refers to if they show samples of their contribution. Lesser known circles will often use this to piggy back on more familiar circles to get their name out in the open more.
You can change your twitter username as it appears in the URL (done this many times). While most people don't do that so as not to confuse followers, the fact remains that it can still change.
Pixiv
I don't know of a way to get someone's pixiv username unless they link to a pixiv blog (which has their username in the URL). If you can get a hold of that, it would a very good idea to use it. An alternative would be to use the artist's Pixiv ID. This won't change unless the user straight up quits Pixiv. The side effect is that there's no quick way to differentiate users with conflicting names, save viewing their member page directly and checking their display name.