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.
Red XIII joins in Midgar when they go through Shinra headquarters. He should have already been available as a party member. He briefly leaves your party when you get to Cosmo Canyon (for like 10 minutes) but then he comes back.
IIRC, Yuffie can be found right after you get a chocobo and go through the Mythril Mines, so it's possible Yuffie could join up in Part 2 since both Kalm and the Mythril Mines don't take up too much time.
Cait Sith though? Nah, no way. That would involve getting all the way to Gold Saucer which means, Kalm, Mythril Mines, Fort Condor, Junon, Costa Del Sol, and Mt. Corel would all be completed. If they are going to have the parts be smaller I'd assume that they would stop at Junon, as the fight with Jenova would make for a pretty good stopping point.