Great circle route seems most realistic...it's just a game, there are no dispatchers cutting a release based on most favorable winds, temps, rides, etc
If you have more than 1000 airplanes on the add more flights page it will not let you do anything. Filter by type to reduce the number and try it again.
I had that issue when I tried to add flights from the "ALL" aircraft page. If I sorted them by type, "757-200" when it refreshed to the only 757-200 add flights page I could add them.