Yeah, I agree with you that they should have made some of the Avalanche gang party members just for part 1, would have been cool to have Jessie on your party.
Hmmmm, Red XIII joins in Cosmo Canyon doesn't he? Cosmo Canyon is 7 chapters from where remake part 1 ends, not sure if the next game will make it that far if future parts really will be smaller than part 1. I could see them adding Yuffie in part 2 for sure, I suspect that she will no longer be an optional party member and they will find a way to work her joining the party into the story. Possibly Cait Sith as well, depending on just how many chapters part 2 covers.
If Nomura was misquoted here and future remake parts are the same size as part 1, 6 chapters each, I would assume that we will get Yuffie and Cait Sith in part 2, Red XIII, Vincent, and Cid in Part 3.
If Jessie was a party member, her only ability would be to hit on Cloud. Would not have been very useful.
As for Red, he actually joins you in Midgar in the original. Basically the point in this story where he joins you as a guest character.