you trade using the global trading station (GTS) or trade with someone that you know that has one
no because you can die
sorry but you have to get another diamond/pearl/platinum game and trade the other starters from there.
No. You can only get 1
No, but if you catch all the sinnoh pokemon; Prof. Rowan will offer you one of the three Johto starters.
All the starters are equally good.
That's impossible.
To obtain all three starters you need to trade with someone.
all three starters and some other high level Pokemon that you've got mostly good moves though not necessarily types.
in my heart
It is not possible to get all three starters in Pokemon LeafGreen without trading. This was an intended mechanic to encourage interaction between players.
Starters are intended to be rare, one-of-a-kind Pokemon. That's not to say there hasn't been a main series Pokemon game that has allowed you to get all of the starters, though - in Pokemon Yellow, you could get all three starters available at the time.
practically any water type. All the water starters can learn surf, palkia can too!