I don't think there's been a tale about what happens if they collect all the Misters (or the second series) but I also think a couple of the Misters are literally impossible to keep contained so there's also the fact that they will never actually be able to complete the collection.
If you're confused about his Euclid classification every sapient scp need to be considered Euclid even if they can be classified Safe because well, they have roughly or higher human intelligence so you never know what they could do.
@Belfegrad
no, Angibot is right on this one. There may be exceptions in the earlier ones before they came up with the rules for the classifications, but sentient beings are considered to be atleast Euclid. It isn't a matter of if they need to feed it as there are safe ones that need to eat that only have the intelligence of an animal.
As a note, any SCP that's autonomous, sentient and/or sapient is generally classified as Euclid, due to the inherent unpredictability of an object that can act or think on its own.