You have to proceed far enough into the story to get to Poni Island. From there, you have to enter the Steelix house and exit out the side door. There...
Have your starter evolved all the way, then breed with Ditto. Trade that starter for the other starters. Or go on GTS and trade whatever the other person wants so...
Nope. Sorry, but most one-time Pokemon (such as Rotom, Heatran, and the Lake Guardians) will never show back up again if you accidentally knock them out. Newer games do address...
Yes. Until you receive the National Pokedex, you will not be able to encounter Rotom. This holds true for Pearl as well, but not for Platinum, where you can battle...
You can only obtain it via special event/mystery gift. However this game is no longer supported for events and thus Celebi is not obtainable without transferring it from an old...