Mine normally stay together until just on 26 weeks up until that time I dont think the gilts come into season. Re naming them, it makes no difference, it hurts just as much if you take pig and pig and pig to the abattoir as it does if you take Colin, Susan and Mary. Had a lot of experience at this, it gets a bit easier the more you do, but it never becomes easy. Then thats all part of pig keeping.
I should explain our reason for naming pork pigs is that both OH and I do the looking after, so if one is not up to the weather its much easier to use a name to qualify which pig needs watching rather than pig 1 or big pig or not so big pig.