I would say espio tbhBased on the designs of what we have seen, I think characters are locked to their cars, but we can probably heavily customize aspects of them.
If only one Chaotix member gets in it will absolutely be Vector, but there are many more choices for large characters to pick from now. Zavok is a likely candidate especially considering he is a bad guy and that fits with the team focus.
But If I were to be bet, you are getting both.
I don't think omega , cream, and charmy are going to be in this game.