About Scooter, yeah I forgot he is a human muppet, but I have always thought of him as a gopher muppet not a human muppet.
About Ernie and Lew Zealand, do not forget they also appeared in "A Muppet Family Christmas" together. In fact, there was a particular scene when the Turkey showed Big Bird to the Swedish Chef as a potential bird feast. As this happens, we see Ernie and Bert with Big Bird and other muppets talking to each other, and we see Lew Zealand talking to Big Bird and (I think) Prairie Dawn, but we also see Ernie and Bert looking at Lew Zealand, Prairie Dawn, and Big Bird talking to each other, so there you go. So, in total, they appeared in 5 movies together. And, another reason why this could work is that Ernie was currently performed by Jim Henson, and Lew Zealand was performed by Jerry Nelson. They look nearly identical, they are similar in personality, and they both have a toy that they like to show off and play with (and maybe some other similarities), they just need to meet each other.
Muppetmonster