The question came up on a German forum about what belongs in a complete collected edition of Spirou. It’s a bit tricky, since it shares a universe with several other series, and if you count every time Spirou or Fantasio makes an appearance in e.g. Gaston, they would almost end up as supporting characters in their own book.
Wherever you draw the line, there will be borderline cases that you could argue both ways. Fortunately, Spirou Reporter doesn’t need to make those decisions, and will happily post this half-page gag (from Capturez un Marsupilami!; Marsupilami #0: “Catch a Marsupilami!”) even though officially it counts as a Marsupilami story:
Why is this a Marsupilami story, you ask? Well, mainly because Franquin seems to have numbered it as such, and because it’s from 1981, long after he gave up the Spirou & Fantasio series. Incidentally, as far as I can tell this was the last time Spirou appeared “live” in a comic by Franquin.