To get the most out of your youngsters they're best put on the darkness BUT bare in mind darkness is not a system, tis just a way of ensuring they're in better condition than if you kept them natural(moult). There's two main ways of racing babies, one is to split the sexes as they reach maturity and run them together the day of basketing, the other is to encourage them to pair and race them paired. I guess the deciding factor for yourself is how often you can train them, racing them paired requires more training.