The teleport thing seems pretty straightforward to me… the ones in Lambda Core were quantum entaglement, too (they had to be for local teleportation, according to Mossman). The Combine’s teleport relied on wormholes, and it could only travel across universes connected by the wormholes. Only Kleiner and Vance had discovered the entanglement technique, so the Combine didn’t have it. Till Mossman showed up and built them a new one (which is the one used in Nova Prospekt).
Obviously ANY teleport requires a ton of power, but the only wormhole one seen – which evidently requires a much more massive power source – is in the Citadel during the fight with Breen. And it’s got a whole Citadel to power it!
My retconning of Race X: They were just some other random race that showed up with the RC, but they didn’t have a giant robo-baby wielding telekinesis to hold the portal open, so they didn’t get to be in the Portal Storms… When BM was nuked, they all died. Poor fellas.