Hi, just discovered this site the other day...

IMHO, yes, Barzini definitely would have had Tessio killed, but not until after all the other Corleone loose ends were tied up:

- Clemenza, Rocco & Neri would have been killed immediately after Michael, if not at the same time. All way too dangerous to keep around & not particularly valuable;

- Carlo, harmless though he was, would be next in line, quite possibly with Connie. This would most likely be handled by Tessio, who would be next;

- Kay & the kids unlikely to be touched (unless it was decided to kill Michael while he was with them), but probably threatened. Likewise Mama Corleone;

- Barzini would have tried to keep Tom around to keep the Corleone business in as much order as possible while he assimilated it into his own. Just how valuable Tom made himself during this time would have determined whether or not he survived afterwards. Refusal to help, well, that's a no-brainer, pardon the pun;

- in terms of an interesting alternative future storyline, Barzini might have let Fredo live & serve as nominal head of the Corleone Family, simply as a puppet. However, I'd like to think that after years of humiliating servitude to Barzini & the other Families, Fredo would eventually find his strength & wipe them all out, possibly with Tom's help. This would be the one flaw in Barzini's plan, underestimating Fredo's loyalty while overestimating his weakness.