I know you've planted now, but I don't think there
is any harm in keeping the top at 6ft. You'll lose some production that a
taller tree could provide, but it shouldn't cause the tree to "bush". Your
pruning practices will make the difference. Once your tree hits that 6ft.
mark, don't prune the leader. wait for it to set fruit and bend under the
weight. "Crop and Flop" is what they call that. The leader may fruit
at 8ft. tall, but will bend down under the weight. The following year,
select a new upright leader and prune off the flopped one. It controls
vigor in the top of the tree. The trick is keeping the leader from
breaking. You'll need to prune enough apples to keep the weight just
right.
If not for fireblight, I might have had some
flopping this year :(