I want to change my partner to a legendary pokemon?

After graduate from the guild (Post credits) and achieve the secret rank (beat blizzard island) many new jobs appear and you could check other walktroughs section of aftergame things to do. There are many legandaries to recruit. Some require the mystery part or similar items. Some just await you in dungeons deepest parts. Some that you find during story mode can be recruited by going back and defeating them twice. After you graduate from the guild Chimcho allows you more freedom with your team. You can make other Pokemon the leader and play as them, you can put you or your partner on standby. This allows you to play as the legandary Pokemon, not just with them as a partner. If you ment when you start the game then thats not possable. I don't think it's possable even with cheats! Maybe recoding but I don't think that an action replay would work.

I don't think that's possible but you can add them to your team after the credits.

Actually there are ar codes for it. You need the change your partner code and then a code separately for the certain pokemon. It works I used it. I wanted to be a Shinx on explorers of time and have myPartener Buizel. There's a change hero code too!