You will need to trade to get one of the three Hoenn starters you did not pick (Treecko/Mudkip/Torchic). However, from my experience you can get them fairly readily from Wonder Trade, but if you don't want to gamble then just use the GTS.
Yes, all starters are required. You will have to trade with a friend, use Wonder Trade, or use the GTS to catch them all.
You will need all Legendary Pokémon except the Mythical Pokémon. These are Mew, Celebi, Jirachi, Deoxys, Manaphy, Phione, Shaymin, Darkrai, Arceus, Victini, Meloetta, Genesect, Volcanion, Diancie, and Hoopa. You will need all of the other Legendaries.
In order to complete the National Dex you have to have owned at least one of every Pokémon species at some point in your game. You do not need to have every single Pokémon at once. (e.g. you don't need a Venonat and a Venomoth at once, you can catch a Venonat and then evolve it.) One thing I suggest, especially for getting Legendaries, is to trade Legend-for-Legend. Put a Dialga on the GTS, ask for Kyogre, get Kyogre, put Kyogre on the GTS, ask for Tornadus, etc.