Some of my series of books, usually children's books, have "special editions" (or sometimes even "super specials"

). I've been trying to figure out how to handle these in Calibre.
Right now I just have them as a separate series. I'm not sure if I like this. I thought about using the decimal points (e.g. 4.5) to put them into the order they appeared in the main series but I'm not quite sure about that and it gets messy when mixed in with omnibuses.
Any other suggestions?