Agree with Julie S - esp as they are now 6 months old, no need to treat for anything at this stage - a wormer would only kill what's inside - it is not a preventative, and if you are about to take them for the chop then worming would be of no benefit - if they have them already killing the worms a couple of weeks before the boars will not improve the boars unless they are looking really thin they probably don't have any serious no. worms anyway.
With modern meat hygiene service present, and lots of checks at Abattoir, it is highly unlikely that you will get the wrong pig back, stories of this tend to be older, and whilst i am sure it does very rarely happen through cock-up, I would not worry about it.
If you are collecting as half pigs to butcher elsewhere, if you slapmark, you can see your mark on the shoulders of both halves and this would show it is yours - the pig is split in halves, one half has the head on, the other not!
If you are tagging, then one half with the head will have the tag on, and you can easily see that the other half fits ! even with a number of pigs matching is easy.
If the abattoir is butchering, slapmarking would only prove that the shoulder joints are yours. Tag, and it will only prove that an ear is yours!
By the by, I suspect you will not want to take all 6 at once, as you will end up with a significant amount of meat. If this is your first time, then take two max on first trip and see how much you get back before taking more - you get an fantastic amount of meat from your pigs, and finding a home for 350 bags of sausages (6 pigs worth) may take some doing!