Yeah, what the title says. There aren't really any "rules." Just know: unless you're doing a reference to something, you need to make the thing Gerson says he is sounds similar to the word "old." Otherwise, we might get some low-quality remixes. It's not so much a rule as it is a piece of advice that I strongly encourage you to follow for the sake of the chain's quality.
All rights in this project belong to their creators. Also, fair warning, beware of Madoka Magica trigger bait. I couldn't help myself.