Go find out the version exclusives and you will know what I mean. Like I said before, do some research and you will know what I'm talking about.
I have my pokedex completed (minus the unlreleased legendaries: Shymin and Arceus) and the only significance that Fire Red and Leaf Green played were being able to catch different pokemon in the wild
If you have LeafGreen certain pokemon would appear, same with Fire red. Its called "insertion pokemon"
but corresponding with Diamond or Pearl? I haven't heard of that before
in any case, even if you miss out on the exclusives of FR/LG or R/S/E you can always trade for them on serebii