If we commmission artwork from someone and it gets posted here with characters we made up, are we allowed to add character tags to it, or are those reserved for well-known and popular things? I mean tags like "firstname_lastname_(character)".

Yes, you can add your own character tags.

As the commissioner and character creator, you have the authority to establish their identity(ies).

Good to know. Just wasn't sure how it fell into the "Tag What You See" rule if the character isn't already well-established before the art is published.

People sometimes want to take TWYS to an extreme and not incorporate ANY outside knowledge at all, which is a bit silly considering that you have to "know" what something is before you can add the appropriate tag. For instance, if I personally had no idea what species a character was in a picture, that doesn't mean that I should take it upon myself to remove any species tags I see that I don't recognize. Given this, that also means that the appropriate species tags shouldn't be prevented from being added.

"All the evidence is there in the picture that this is _________" should be your thought process when tagging. If it looks just like someone's OC, then it should be tagged as such. If it looks just like a certain species, then it should be tagged as such.

Just be wary. If you've commissioned a male character with very feminine features, and you happen to commission an image of him from behind where there is no evidence of balls or penis, don't get worked up if it is (correctly) tagged ambiguous_gender.