Is it possible to have different categories of favorites? Or some way to save groups of things. (If not I suppose I'll just use bookmarks)
I think I know what you mean. I have seen websites that offer an accounts gallery the option of creating sections much like the program Windows Gallery even though the posts are uploaded to a 'main' gallery. You are correct. You would have to create browser bookmarks.
Safebooru does not currently have the 'add to pool' option on posts.
The difference is that this site is like a giant, single account-gallery; the user doesn't post to their own account, the post goes into the main 'account-gallery.'
Perhaps , in the future, and if the number of people actively using Safebooru increases, then a 'pool'ing system can be put in place... I certainly hope so.