On Pokemon Pearl's title screen, if the Pokemon LeafGreen game is in the GBA slot, click on "Transfer From LeafGreen" and you can choose six Pokemon from LeafGreen to transfer to Pearl, but be careful-you can't trade them back.
If you trade Pokemon between leafgreen and ruby ruby will have your leafgreen Pokemon and leafgreen will have the ruby Pokemon both in the party they will not be sent to the PC unless you personally walk to the PC and do so.
Pokemon cannot be transferred from Diamond to Leafgreen. This can be done backwards, however as Ruby, Sapphire, Emerald, Firered, and Leafgreen Pokemon can be permanently transferred to Diamond, Pearl, Platinum, Heartgold, or Soulsilver.
Oddish is a FireRed-exclusive Pokemon in Generation III. To obtain an Oddish in LeafGreen, it has to be traded. Oddish is a FireRed-exclusive Pokemon in Generation III. To obtain an Oddish in LeafGreen, it has to be traded. Oddish does not appear in LeafGreen.
You cannot catch Registeel in Pokemon LeafGreen. This is due to FireRed and LeafGreen being able to trade with Ruby and Sapphire. Pokemon Ruby and Sapphire have the three legendary golem Pokemon including Registeel.