Yeah, it's definitely the good ol' destiny thing. Aside from the fact that only 6 vs 13 Xehanorts could be quite hard (read as: suicide). They can be more, but less...
Still, it could be fun analyzing who could be left on the bench:
- Sora, Riku, Aqua, and Mickey are, well, necessary manpower. Probably even Lea, and he also has the "I gotta save my friends reason", and he has to have the anime fight with Isa.
- Terra, Xion, Roxas, the Chi characters it's unclear where they'll be or if they're take part at all.
- Kairi, I guess it'll be too dangerous to leave her? Considering she's also a Princess and kidnapping her would probably assure the other 7 lights?
- Ventus, Donald, Goofy (since they could be Guardians too) are probably the most expendables: D&G are strong enough to not be kidnapped easily, weak enough to not be necessary. Ventus is strong, but seen the history with him and the X-Blade keeping him out would actually be the safest route.
Problem is, he showed how good he is at staying put, and "D&G are Sora's strenght" is a must.