@DepressedSoviet, you might even say making it convenient to have such crossovers is what the Rifts world is all about. But surely, we can come up with something a little more elaborate than just "They came through a rift." The features that you want the rift to have are going to depend on the role you want robotech forces to play in your campaign. Me- I've got a dozen plans to conquer Rifts Earth and single-handedly save humanity from the Coalition States, the vampires, the Xictix flying bug guys, the Splugorth, the Mechanoids, and any other comers. And it turns out the Expeditionary Force was designed for a planet-hopping campaign. According to The Shadow Chronicles book, they built an SDF-4 and called it "The Liberator."
I don't really think the Invid are as powerful as the Mechanoids, and the Splugorth and the flying bug guys might have extra-dimensional resources that would make them competitive too. But your point is well taken. If the Regess were to show up with her entire army, she would crush Rifts Earth just as fast as she crushed The Army of the Southern Cross. But if the Mechanoid mothership orbiting Gideon-E were to show up, she'd be well-advised to do another Symphony of Light and get the hell out of there.
I don't really think the Invid are as powerful as the Mechanoids, and the Splugorth and the flying bug guys might have extra-dimensional resources that would make them competitive too. But your point is well taken. If the Regess were to show up with her entire army, she would crush Rifts Earth just as fast as she crushed The Army of the Southern Cross. But if the Mechanoid mothership orbiting Gideon-E were to show up, she'd be well-advised to do another Symphony of Light and get the hell out of there.