I have two- and three-year trees on M9 rootstock. I followed all the conventional wisdom which said to head back 450mm or so above where each scaffold was to be set.
I now work for a large (culinary) apple growing, packing and exporting outfit and had one of our orchard gurus have a look at my trees. He pointed out a number of things:
1. The commercial guys are about getting their trees up. Pretty much, they want to get close to terminal height as soon as possible. They don't therefore head back. This helps develop a thicker leader which means it can support fruit sooner.
2. To keep the tree growing upwards, they remove any laterals larger than a third of the thickness of the leader. I had been keeping these as my scaffold branches and congratulating myself on how strong they were. Of course, a lateral of the same (or close) thickness as the leader thinks its a leader too, so the tree acts like a multi-leader. Commercial guys select and tie down their laterals about year three.
3. I have found most cider cultivars are basally dominant, meaning that the lower scaffold branches want to become leaders. My experience is that they need tying down sooner than culinary cultivars so that the leader gets a chance to get ahead.
4. Keep apples off the trees (by removing the blossom) for the first three years. This ensures energy goes into growing the tree and not the fruit.
The bottom line is cider cultivars behave differently from culinary varieties and they have different characteristics between each other, too. Knotted Kernal, for example, is very compact and upright while Yarlington Mill is quite leggy. Looking at the two, I would leave KK to its devices but use closer spacings but give YM plenty of room. Talk to people who grow cider trees if you can, particularly if they have the varieties you do.
If I have confused you, then you are well on your way to becoming a cider apple grower!!!