Yeah, Karn did make the plane of Mirrodin, though he called it something totally different. There was one other sentient life form on the plane, and that was his little warden. Karn left to planeswalk with Jeska (yeah, she's a planeswalker, too) and put the warden in charge. The warden twisted things and abducted other life forms, and he himself became Memnarch.
Oh, and the Mirrari--it was also created by Karn. Apparently, there were a lot of them, and Karn sent them to different worlds to observe those worlds. Basically, the Mirrari, though corrupt and super powerful, was just one of many probes.
We will likely not see Karn again except as a segue device as he was last seen.
As for the other representatives, I specifically remember Yawgmoth's phantom for black, and I am fairly certain Kamahl was green. The others, I'd have to look up if I can find my dang Scourge novel.
