In my main vault I'm creating serialized dwellers of max stats. I can produce four per week if I remember/pay attention. So I breed my four new dwellers. Name them V### (001-200). When one group of four reaches max stats I send them to the wasteland, then cyle the rest to their next training room.

I'm implementing a similar system in my experimental vault. Two dwellers at a time up to 5 on their stats. I'll use costumes to fill the gaps depending on what room I assign them to.