While the Kryp & Tonn story may be only legend, the colonization of Krypton from outside is supported by Maggin's Superman: Last Son of Krypton and Tales of the Legion of Super-Heroes #325 (Mon-El tells Ultra Boy Daxam colonized Krypton).
What exactly did Mon-El say about Daxam colonizing Krypton specifically?
I have to paraphrase since I don't have the book at work, but when Jo asked which world was the original and which the colony, Mon said they (the Daxamites) might be accused of chauvinism, but he was taught in school that Daxam was the parent world. Dev-Em -- still a Kryptonian survivor in this era -- woke up from K-poisoning at that moment to argue, no way he's claiming Mon-El as his long lost daddy.
Green Lantern #35, Vol. 3 (Jan. 1993) describes the Great Humanoid Diaspora of around 5 million B.C. This helps explain the compatibilities of most DCU human/humanoid races, and may be related to the Daxam/Krypton link above.
What was the "Great Humanoid Disaporia"?
Basically, this was a seeding of worlds with humanoids in this era. That means most if not all humans/humanoids came from a common source dating back to this event.