First time I play is Lulu but second time is Rikku.

I can switch them. Be sure when battle occur, if they get hurt just heal them. Best way use this method.

If you want Rikku ride mobile machina with you. Make sure when encounter with fiends. Rikku cast "Provoke" and Tidus use "Guard". When Rikku get attack, Tidus will block it. Or when Rikku get low HP, let Tidus heal her.

This will affected to Tidus' Overdrive "Blitz Ace". If you proceed method above. It might be Rikku who throw the ball to Tidus.

Sorry, there is not going to happen ride mobile machina with Yuna.