As far as I'm aware, it's perfectly okay. The collection thing, I guess, is to help give credit to the artist? I dunno. But there's no rule against posting others work done for you, as long as you credit them.

It's technically okay for a user to not use the Collections system. We strongly encourage using the Collections system, but it's not mandatory; still, you need permission from the artist.

I. A. 2. of the Community Guidelines: Original Content Collections: If a particular work has been uploaded to the site and was created for you or features you, your character(s), or your intellectual property, then the original uploader may offer it to you as a collection. If you have permission from the artist, you may upload it to your gallery, but the use of the collections feature is strongly encouraged.