It will be more stress on the affected pullets to introduce them to the bigger established birds. Unless they have a lot of room and /or protected from the larger birds bossing them. If it were me...I probably wouldn't introduce them until everything was under control. Unless they are terribly crowding or having a hard time where they are. That would change my decision.
I've never used sand. I'm probably not the best one to give advice on that. I can agree that having them on a clean area or clean bedding so the pullets can't continue to overload themselves by ingesting infective oocysts is beneficial. I use shavings. Every time I add developing chicks from the brooder, I bed a layer of fresh bedding.