Michael told Clemenza and Tessio that if they sat tight they would be able to have their own Families. Clemenza did. Tessio thought he couldn't survive until then. And ironically he was right!
But what would Michael done if Clemenza and Tessio had both turned traitor? Would the Corleone Family have survived after Vito's death?

Since at the time they were the only two family capos (possibly plus Lampone), their joint efforts would probably have toppled Vito. Working together or separately, they both had plenty of opportunities to murder Vito.

By the way, one of the most egregious errors that both Puzo and FFC made was to have the protagonists speak any words about forming their own families. There is nothing in Mafia history, anecdotal or not, that lends itself to an American Don or capo of a Cosa Nostra family being able to create a new Mafia family. While there is plenty of Cosa Nostra history about capos taking over families (a la Anastasia, Genovese, Colombo, Galante, Gotti), there is nothing that I am aware of that sanctions a Don creating a new family out of whole cloth.

This is an interesting question, Lilo, but it's so improbable that it's difficult to answer:

First: any caporegime(s) or other conspirator(s) can murder the Don and attempt to take over. The issue is whether or not they have enough support in the family (and maybe in others) to stay in power. Even a cocoz' like Gotti consulted factions in the family, and key players in other families, before he had Castellano whacked, and Big Paul had given them plenty of provocation. As long as Vito was alive, he was all-powerful and commanded tremendous loyalty.

Second: The Corleones' top strength was Vito's political power, which seemed to be held exclusively by Vito. He was attempting to transfer it to Michael before he died, and I think his phrase (in the movie), "this wasn't enough time, Michael," reflected his fear that Michael would make his move .before he had all of Vito's political power "wired into him" (the novel's phrase).

Third: As Oli notes, creating a new family out of whole cloth is unprecedented and improbable. If Vito had given them permission, what would be left for Michael, the appointed heir? I'd add that there's never been a co-Donship in Mafia history, and never will be. Clem and Tess either would have formed two families out of their own territories, or fought each other to the death for sole proprietorship. Either would have fatally weakened both. I posted that in a thread:
http://www.gangsterbb.net/threads/ubbthreads.php?ubb=showflat&Number=574490&Searchpage=1&Main=20369&Words=%2Bstroke+%2Bof+%2Bgood+%2Bluck&Search=true#Post574490

That's why I believe Tessio's betrayal was a major stroke of good luck for Michael.

Vito correctly assumed that one person would betray. It just seemed like he and Michael were taking a big risk. As you say Michael was actually lucky.

There's a strange little dynamic: during the drugs war both capos were found to be past their prime, and Vito dies several years later. So Michael might have welcomed the chance to replace either one. Their were risks in replacing either, because they had their own loyal people under them, so removing one capo would almost certainly require the assistance of the other.

The book is pretty melancholy about the family leadership. Vito is slippin', he knows Sonny isn't focused enough to be his heir, Tom isn't a war time consigliere, the capos are too old to command.

Towards the end of the story Michael is building Rocco Lampone Regime and possibly Al Neri as well (although this is a guess) we can assume Michael was developing his own loyalist in the Family. He had probably started with placing roots in Arizona and getting a foothold in Vegas.

Its far to say Michael would have been able to survive on the West coast and leave the NY rackets in the same way of GF2 but it wouldn't have been on his own accord.
Of Course, Michael's power is secured until the end of GF1 and expand inbetween GF1 and GF2 but I think he would have survived.

@Turnbull
It hasn't been an official "co-Donship" but there have been several powersharing dynamics in history (Paul Ricca/Tony Accardo, William Tocco/Joe Zerilli, Tom Gagliano/Tommy Lucchese etc) Its not out of them realm of possibility that Clemenza and Tessio could have worked together. Tessio surely could be negotiated with But I don't think Clemenza truely wanted to seperate

Yes, and that's an important point: The novel tells us that Vito kept Clem on a short leash, but allowed Tess to seem as if he were an independent operator, in part to keep his enemies in the dark about whether Tess was really part of Vito's armament. But, I believe Tess as a result developed an independent feeling that Clem may not have had. In the fish tank scene, it's Tess who starts the complaints. When Vito says, "Do you trust my judgment?", Clem says, "Always, Godfather," but Tess just hisses, "Yesss." And, when they leave the room, Tess gives the left-handed handshake--sure sign of impending treason.

That's why I think Tess would never have accepted being under Michael when he moved to Nevada--and why Tess's treason was a stroke of good luck for Michael.

Of course, Clemenza does not shake Michael's hand at all. However, he does shake Vito's.

Didn't Sonny also have a crew?

I don't think so. He was, in effect, Vito's underboss, and so could boss Tess and Clem. After Vito was shot, he chose Tessio's men to bodyguard the compound because he wasn't ready to clear Clem at that point.

Sonny did have a crew. Smaller than Tessio and Clemenza. From the novel Don Vito had territory in the Bronx (Clemenza), Brooklyn (Tessio), and Manhattan which that capo was never identified. We are never told who all the capos are, only that Tessio and Clemenza were the old guard who Don Vito trusted. Vito making his son Sonny from Soldier to Underboss would be egotistic which Vito was not. So it stand to reason that Sonny was a young capo he was born in 1916, before he became underboss to his father. In the book we know Michael with Vito's support and blessing made Rocco Lampone capo was bolstering that caporegime secretly making it the most powerful in the family. Al Neri, I have seen posted as a Capo, but he was direct with Michael and had no one under him in a crew. He was in fact Michael's Luca Brasi, and could do more than what Brasi could do.

Glad I found the book, I was wrong, Al Neri took over Tessio crew. Rocco Lampone is a capo and considered Michael most trusted Capo. Only Barzani and Tattaglia are killed.

Sonny's position in the novel is murky. A few hints:

1. He did have some sort of crew. They were disbanded, I believe after the peace conference, and placed in the others' regimes;
2. He is the only one in the Sollozzo meeting with Vito and Tom;
3. Vito was considering him for underboss at the time they moved to Long Beach (late 30s?). As illustrated by the incident with the "furnace inspectors," Vito wasn't happy with Sonny's performance; and,
4. When Vito is shot, Clemenza calls Sonny and says something like "You have to carry the ball, Sonny," which he might not have to say if there was a clear chain of command.