I want the best leagues for finding the best young players. Also do you have to have them on playable for the players to show up on the search?
While adding leagues will load more players from there, the more leagues you add the slower the game becomes; especially if you set them as playable.
Playable or not makes no difference to the amount of players loaded.
You can fiddle with advanced database size settings to load more players in a more efficient way in terms of game speed. For example, a save with 5 playable leagues and 50K players is likely to be slower than a save with 1 playable league and 100K players.