The point is that one rules set can't address all genres, at least not very well. The d6 rules, in their simplest form, are best suited for fast-paced action in realms like the Nile Empire, Aysle and Core Earths of the Schwarzenegger flavor. The Living Land, Orrorsh and Nippon Tech need something that slows players down and makes them think things through. The Cyberpapacy is a tough call: fast-and-loose on the surface, but the overall tone is more oppressive and deadly. I can't imagine swashbuckling pirates on Magna Verita, for instance. I have no advice on Tharkold, except not to use it in your game.
World Laws are an important tool for tailoring the rules to a Realm. As you might guess, I suggest a new set of World Laws for the Living Land and Nippon Tech. I've tried to power down Possibilities, to make it easier to present problems the PCs can't spend their way out of. Avoid the temptation to add Laws just because they're cool; keep an eye toward the pacing you want and the things you want players to spend time on.