The game only starts 2 years before ANH. A campaign to take over/liberate the galaxy will take time and it really doesn't make sense to have all available unit and ship types available from the beginning. New units and ships will get researched as the game progresses. I don't know if the player will have control over the research or will just be given new technology. Missions such as stealing the X-Wing prototype could provide the X-Wing technology earlier than it would come if the mission was either not played or unsuccessful. This kind of mission could also apply to other units and ships too.
Well i know that if you say Ryloth is confirmed or we find out it will be, there has to be twi'leiks. Think about it what would be on the planet then? It would be like having tattoine with no jawas, sarlacs, banthas, and tuskens.