ofcource you can
No.
no
the only way possible is to get a wireless adapter or link cable(Pokemon Centre does not exist in firered or leafgreen
you can get Pokemon that are not in the other version
There is no emerald to trade between firered and emerald you need wireless adapters.
You cannot "trade" pokemon between Pokemon FireRed and Pokemon Pearl. Instead, pokemon from Pokemon FireRed Version can be transferred to Pokemon Pearl via Pal Park.
Pokemon Red was the original Pokemon game on the GameBoy along with Pokemon Blue. Pokemon FireRed is the GameBoy Advance remake that is massively updated. FireRed has all the new Pokemon introduced since Red and Blue along with new mechanics, better balancing and significantly updated graphics.
Firered and leafgreen both can catch different Pokemon but there pokedex is the same, here is some examples: magmar only in leafgreen not firered, oddish in firered not leafgreen, ekans in firered not leafgreen, bellsprout in leafgreen not firered, pinsir in leafgreen not firered.
you need a special link cable though
no you cant its an old game and a new game
The only way you can trade between emerald and firered is by wireless adapters, in emerald you need the national dex and in firered you do as well plus in firered and emerald you need to beat the elite four and in firered you need to deliver the ruby and sapphire to celio at one island once this is all complete you can trade between emerald and firered any Pokemon you want.
To battle Pokemon in Fire Red/ Leaf Green in Emerald you need to beat the Elite 4 and gain the Nat. pokedext. then you can Trade and Battle Pokemon using the trade and battle mode opptions in the Pokemon center. Some trainers have Pokemon from Fire Red/ Leaf Green.