Can you duplicate Pokemon in the LeafGreen and FireRed versions for Game Boy Advance SP?
No you can't, you can only breed Pokemon that are able to be bred.
Why can't you trade Pokemon from crystal version to leafgreen firered rubysapphire or emerald versions?
You can use the original Game Boy Advance games in the DS/ DS Lite Slot 2, however you will not be able to trade with other FireRed/LeafGreen players. To do this you need a Game Boy Advance and the cables/ wireless attachments. This is for FireRed/LeafGreen only, not the original Red/Blue which were for Game Boy/ Game Boy Colour. These older, large cartridges will not fit in the DS.