You can only get them through trade.
You can't catch them, but you can certainly trade them from other games.
you have to trade them from other Pokemon games
This is possible in the Japanese version of Pokémon Crystal.
You can only get 1 which is decided by what starter you chose. To get the other 2 either have another firered or leafgreen give you the other 2 or take the purified ones from Pokemon colosseum.
No.
Through trading from other games yes.
You have to choose Charmander as your starter and evolve it twice to get a Charizard. Only other way is to trade with someone that has it.
Do you have any other game cause if you do you can trade your first starter into that game then erase your game then choose another starter, there is no way other than cheating which you shouldn't do that you can get all 3 starters other than trading from firered.
you can if you chose it as your starter but the only other way is to trade one from another leafgreen or firered.
Your rival will get a venusaur only if you chose squirtle as your starter i believe he will get one when he is at silph co, other than that to see or get a venusaur a friend with firered or leafgreen needs to give you one.
look you go to digglet cave and get out of it in the other side then there will be a tree that needs cut after you cut it there is a building beside it go to that building and there a professor that asks you if you have 10 pokemons in pokedex if you do he will give you flash
there aren't much differences, the differences you can spot are some pokemons change. eg. in fire red you get oodish, gloom---in leaf green you get bellsprout ,veepinbell instead there are also others like for growlithe you get vulpix instead you get slugma instead of magmar in leaf green and the pokemons of the elite four also changes in both the versions there are also many other pokemons you get in leaf green which you don't find in fire red.