We have never seen Donald or Daisy at Plaza Inn. We have seen Pooh almost every time we have been. We have always seen some Pooh Character there- if not Pooh then Tigger or Eeyore. We have seen Captain Hook, the Fairy Godmother and Minnie every time we have been. We have seen either Chip or Dale or both every time we have been. The characters do have variation there, so none of those are guaranteed.
We were there last week and saw Minnie, Captain Hook, Fairy Godmother, Max, Rafiki, Pooh, Tigger, Eeyore, Chip and Dale. We have seen Rafiki and Max there more than one time, so I think they are pretty common. Other characters we have run across, but only once were Gepetto, Pinochhio, Penguin from Mary Poppins, Tweedle Dum, Brer Bear, Mad Hatter, Alice, and I'm sure there are a few others I'm forgetting. Sometimes there are some good surprises there! I would say chances are about 95% you will see Minnie and a Pooh Character (but not necessarily Pooh). I think there is about an 90% chance you will see Fairy Godmother and Hook and a chipmunk. Occasionally, we have seen a character in the restaurant that did not come to our side of the restaurant or our table-- one trip we saw Eeyore outside, but he never came to us inside and we have also had that happen with other characters there.
It is our absolute favorite character meal at DL and WDW. DL typically has better character meals than WDW except that a few months ago they standardized and reduced the characters at Goofy's Kitchen. I think I will cry if they do that to breakfast at Minnie's!!!!!!!