I actually really doubt the possibility of other Ketran survivors. The Ketrans that survived with Toomin took off on their superspecialawesome crystal space ship, and I got the impression that the spacecraft was so revolutionary for them, there was no possibility of there being another one. All the other crystals would have been wiped out when the Ketrans at the docking spars were killed. I guess some might be able to survive by moving to smaller crystals without being spotted or whatever, but it strikes me as a long shot.
There's another possibility, although it might also be a long shot. The whole 'eradicate-the-Ketrans' thing was all just a great big misunderstanding on the Capasins' part, right? So what if, at some point before they finished the job, they somehow realized their error? Perhaps it finally occurred to them that there was no way the Ketrans were technologically advanced enough to be able to toy with the evolution of other species, when they couldn't even fend off the Capasins' attack. Perhaps they picked up enough of the Ketran language (through translator chips or whatever, it's possible) to hear one of the panicking Ketrans say, "It was just a *$&%ing video game! Stop shooting us!"
This exact question is what bothers the heck out of me.
Especially if during the big battle with Crayak, he lost his Ketran body, yet APPEARS TO TOBIAS IN #13 AS A KETRAN!
For one thing, I don't recall him doing that at all in that book. He appeared as a half-hawk-half-human creature (because that's what Tobias would 'identify with'), didn't he? Are you thinking of another book, perhaps?
For another thing, though, I was under the impression that the various forms he took after losing his body were just illusions that he produced, which had nothing to do with anything real. The guy is nearly all-powerful, after all. If he wants to appear as a long-extinct species, well, that isn't any weirder than a glowing blue person (#7), or a half-hawk-half-human (#13), both of which are beings that not only do not exist, but which have never existed.
Does that make sense, or did I just completely miss the point you were trying to make?