No, after you enter you can only try and defeat them.
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once you get all bADGES AND BEET THE ELITE FOUR
Yes you can. Mainly once you beat the elite four, but other more common Pokemon can be found that are not exclusive to the Sinnoh region.
It appears randomly around Kanto once you defeat the elite 4
it dissapears and you get a red scale but once you beat the elite 4 you get to challenge it
Yes, yes you can. Once you defeat the Elite Four in Pokemon FireRed the only thing you have to the is complete a couple of sidequests in the Sevii Islands.