I guess for a round, it doesn't matter as long as it's within a size range.
However, do I have to get custom ring made for cushions since they vary so much in shape?
If so, is it fairly easy for a jeweler to reshape the halo to fit around whatever cushion I decide to purchase?
Yes, if it is a site like WF or BG and they show a stock halo setting, they have to be allowing for the halo to be made to fit the stone. I am not sure that would be the case everywhere, but all the rings I have seen from those have had the halos fit the stone.