We keep OSBs and they all go at 70-80kg, beyond that weight they put on fat too quickly, we don't differentiate between porkers, cutters and baconers, but we still use the meat for bacon, pork and sausages.
We separate the sexes at about 4 months so we don't get any of the problems you may be finding, however, before 6 months weaners do play a lot which can be seen as sexual behaviour.
If you want pigs of the same sex I suggest you put a request on here, there are plenty of weaners around and you will soon find a local breeder who has 4 of the same sex.