The people in White Forest is determined by the different people you invite over from Black City using an Entralink. When you restart your game, you'll have to invite more people, because restarting erases all data from your current game, so at first your White Forest will have a little amount of people.
In short: Yes, you will lose your Pokemon you currently have. But the new pokemon is determined by the different people you invite over.
Here is a chart for the different people and Pokemon.
Hope This Helped :D