Not fit the theme, not interested, or is it more of a power scaling thing?
Either way I can see your point, though I have to ask is their any one of the original ten you wanted to include but decided against doing so as starters?
What so toy mean!? They fill different niches!
Grey Matter:Incredible short and super intelligent.
Kirby's species (or actually even Waddle Dee's if Bandana Dee is any indicator): Incredible short and capable of curbstombing beings much, much, much larger than them.
Now if we really species whose only gimmick is being small I recommend Olimar, they have a really high Oxygen intolerance (Bad considering how many other species seem to breath it), are the size of a quarter (Greymatter would be a giant to them), and I'm pretty sure in this setting the only reason they'd still be alive in universe with Vilgax. the Highbreed, and whatever the crossover adds is because they give sunbusting weaponry out to their delivery men.
I joke of course... I can see why having Grey Matter would make someone not want another 'small' species.