I agree that it would kind of wierd for Kirby to inhale King Dedede or Metaknight; though I don't see Sakurai preventing it because it would be unfair.
I thought about that. One of my ideas is this. You know how your vacuum can start sucking up your rug and you have to violently tug it free? Well, I came up with this;
Since Galaxia or King Dedede's hammer isn't exactly a part of the character per se, Kirby could inhale that instead. Though they wouldn't be pulled from the character's hands, Meta Knight/King Dedede would be struggling to pull it out of Kirby's mouth. If Kirby copies it, as the animation is executed the weapon then tumbles out of him at the same time and instantly back into their hands. Thus it'd still have all of the same properties as before and the characters will be close enough to Kirby as not to result in any complications. (Such as Kirby getting hit away just as he copies it, thus having Meta Knight losing Galaxia.)
The other ideas mostly involve Kirby getting some form of compensation exclusively against Meta Knight and King Dedede, but I rather avoid character-specific advantages so I won't get into that.