Why can't they, why wouldn't they be unable to, it's not like anyone who loves the Muppet Show would see Pepe and Rizzo and say oh well I'm not going to buy this, the fact the you say they wouldn't really be able to use them makes no sense, you're making sound like it's a law or something.
Well I didn't mean that extact picture, that was just used as an simple to get someone an idea of what I'm talking about, actually it should be a whole new group picture.
And besides you're wrong about Rizzo. Rizzo was a part of the Muppet Show, he started changing from just one of the Muppet rats to the famous Rizzo around season four. So putting Rizzo on the set would totally make sense. Putting him in season two was an odd choice cause he wasn't Rizzo than, but around season five, he was and it would be fine if you want to stick to the true Muppet Show.
As for Pepe, well I agree, but he's become so popular that Disney almost always founds a way to sneak him in, I have no doubt he would be in the special features. But I do agree, I would rather see it focus on the "classic" Muppet Show muppets, but Pepe has pop up in the extra features of Muppet Show seasons dvds, Muppet Movie, GMC, and so on. So putting him in a group cover, well whatever.
I understand what you're saying overall though and I agree. It should be all about the Muppet show and the Muppets that were there at the time. But to make it sound like it would be wrong to include the likes of Pepe just sounds odd to me.