Yes. Every single Pokemon from every Pokemon Game is part of the National Pokedex
It is not possible to trade a Happiny to leaf green, not only because a Happiny does not exist in the Leaf Green pokedex but you can only transfer Pokemon from FireRed/LeafGreen/Emerald/Ruby/Sapphire to Diamond/Pearl/Platinum, and not the other way around.
Just like in Diamond and Pearl, you have to have an old DS or any DS because the DSi. You cant trade your Pokemon between them but if you got the gba game in the ds with platinum, you cansend your Pokemon to the DS game. You cannot however get them back on the gba games.
Emerald as alot more features than firered and leafgreen including Pokemon contests, the battle frontier and you can catch certain Pokemon firered and leafgreen both can't get over all if you want to buy a certain GBA Pokemon version get Emerald but leafgreen and firered are good choices to that's because it includes the new sevii islands to explore and the game has a little more difficulty in it that is actually a good thing by the way.
... roses are red because its fireRED leafs are green because its leafGREEN
No because they are two different games and because the codes are different
It is not possible to trade a Happiny to leaf green, not only because a Happiny does not exist in the Leaf Green pokedex but you can only transfer Pokemon from FireRed/LeafGreen/Emerald/Ruby/Sapphire to Diamond/Pearl/Platinum, and not the other way around.
Just like in Diamond and Pearl, you have to have an old DS or any DS because the DSi. You cant trade your Pokemon between them but if you got the gba game in the ds with platinum, you cansend your Pokemon to the DS game. You cannot however get them back on the gba games.
Emerald as alot more features than firered and leafgreen including Pokemon contests, the battle frontier and you can catch certain Pokemon firered and leafgreen both can't get over all if you want to buy a certain GBA Pokemon version get Emerald but leafgreen and firered are good choices to that's because it includes the new sevii islands to explore and the game has a little more difficulty in it that is actually a good thing by the way.
Dive is not an HM in Pokemon firered or leafgreen however if you trade a Pokemon that knows dive from ruby or sapphire or emerald now you have a Pokemon that knows dive But you can't dive underwater in firered or leafgreen because it wasn't invented in the kanto region.
you can't plant berries this is exclusive only to gold, silver, crystal, ruby, sapphire, emerald, diamond, pearl, platinum. the reason why is because game freak wanted firered and leafgreen to be exactly like Pokemon blue, red, yellow, and green with a few changes.
... roses are red because its fireRED leafs are green because its leafGREEN
You must get ditto from emerald, firered or leafgreen because you can't catch it.
Not at all you can get kinda close but you must trade because you can't get ANY hoenn Pokemon in either firered or leafgreen except deoxys by Nintendo event.
IT IS NOT IN POKEMON DIAMOND OR PEARL BECAUSE IT IS ONE OF THE STARTING POKEMON IN LEAFGREEN!!!!! Migrate one from firered or leafgreen.
You can't because mamoswine was made in the 4th generation games diamond and pearl and platinum it never existed in leafgreen.
i recommend scizor because onix exist in both versions and therefore possible for both firered and leafgreen to have steelix. But, only firered possesses scyther and therefore, better for it
No because they are two different games and because the codes are different