Have you not been paying attention?
Having this be the first time I’ve seen that the voice of the smaller alien (which a quick search was Reuben or experiment 625) sounded familiar and sure enough, that same search confirmed that it was Yakko Warner from the Animaniacs (and various other voices). I love that guys voice.
625/Reuben was one of the best comic relief characters in cartoon history.
625 is the best because he can do everything stitch can do but of causing chaos or saving the world, he would rather have a sandwich.