well, i dont think they kill each others because if they did, all the claymores would hate the organisation. I mean, if some1 make you kill humans since childhood, you would become a sikco and hate every ·%&" thing in the world and you wouldnt mind killing some humans(something that the organisation doent want to happen).
Maybe they had some rught training but not to the extend of killing the kids because it is not convinient and beside when the flash back of number 5 apear, she saw teresa escaping from the the organisation for fun, and she wasnt sad, if it would had been so bad, she wouldnt had escaped because of the punishment.
In my opinion, the organisation choose orphan kids because they can teach them wathever they want, they are easy to brain wash, nobody would miss them if they die and alredy hate the yomas. Then, if they manage to do well in the training, they put yoma inside of them and if they survive(probably alot of them die in the proses or aweken and get killed) the organisation keep there training until one of the claymores die.
to resum, With the few that probably survive in the surgery, and with the constant change in the claymores army(because of a fight with an awekening, the black cards, surprises atacks by yomas or get killed by the organisation), a "school" with 300 students is enought. and with the ones that dont lear, or are weak, probably disapiar