to change which Pokemon GO into the dungeons you need to beat rayquaza first but if you want to get a new starter you can either make a new game or try to recruit them at certain dungeons you can find which starter is where using Google hope this helps =D
yes
you cant
no
dungeon leader can be any Pokemon you recruit. you will be able to change leading Pokemon after you graduate .
Yes
After you beat the game, you will have the power to change the leader at Chimecho's Assembly. You can not change the leader in a dungeon, though.
you can't (I think)
No.
you have to beat rayquaza to start evolving and having different leader pokemons.
You can't.
you can't sorry man, but it ain't gona happen!
Yes but after you complete the game. Including darkrai.