no you have to migrate all 3 in order to catch reigegigsis
You catch Regigigas by migrating Regirock, Regice, and Registeel from a GBA game then go to the Snowpoint temple in Snowpoint city.
You can't find Registeel in Pokemon Platnium. You just migrate it in Pokemon Ruby, Pokemon Sapphire, and Pokemon Emerald.
you cant catch it in platnium but you can trade it
No
You can get regigas on platinum by putting regiice,registeel,and regirock on your team. Then go to the top of the temple in snowpoint citie. Then click on regigas and he will wake up! Be carefull when trying to catch him though. He is only level 1!
you cant
Yes you can.
no
no
stark mountain
no ditto is there
you get it from a fossil