Thanks Neil, that's a great help, and yes, it makes sense.Yes Mel. Let the tall, thick ones grow more and keep the smaller trees trimmed harder. There are a couple of things to watch though:
Trimming will encourage more branching. More branching can also = more leaves and therefore more thickening so as well as regular trimming to keep them smaller you may also need to thin out the resulting shoots to keep the smaller ones restrained in bulk as well as height. Hope that makes sense.
As mentioned earlier, trees on the outside of a group sometimes grow faster because they have more root space so watch out for that as well while allowing group plantings to grow.
If all else fails you can still cut a group apart after a few years and rearrange the individual trees or clusters of trees into a better arrangement.
Cheers Ryceman.