You must complete the main game and the post-game, up to the point where you graduate from the guild. You can then put your main character and partner on standby and use a new leader for exploration, this does not however truly 'replace' your hero, you will still have to use hero and partner for certain plot mission.
it depends what partner you want and what Pokemon you count as rare.
any water type
get it as your partner
you can get him from a partner or leader
You either have to be it or chosse it as your partner
squirtle and pikachu will be the best
Totodile, with partner of Torchic
Yes, he/she can be a starter or your partner.
nope
From What I know I believe you can only get it as a partner or your main character in Pokemon Sky dungeon.
nothing. you can recruit another pokemon. if you cant recruit it again your out of luck.
First, beat primal dialga. Then, you can play as any Pokemon you recruited including your partner.