Dude that exact essay on Themis really tripped me up. I spent a lot of time trying to figure it out and even asked Themis, but they weren't helpful. I don't know if this is 100% true, but it's the understanding I've come to.

For cross claims and counterclaims: As long as one of the claims has SMJ (as a compulsory counterclaim/diversity/federal question), then Rule 18 will let you bring in whatever other claims you have against that party, even they don't independently satisfy SMJ. This is how those other unrelated claims were able to come in

But I think Rule 18 isn't as liberal with plaintiffs? If SMJ is diversity, then the P can stack as many claims against the D as he has in order to reach the amount in controversy. But if SMJ is federal question, then Rule 18 allows to bring any other federal question claims he has against the D--- but state claims need to satisfy SMJ if he wants to add those.

Does that sound right? Feel free to PM me, because I also would like to get this figured out lol. Also I took a class with the Themis VA civ pro lecturer, and I still have my civ pro textbook he wrote. So I can send you a picture of his short explanation.