Railing size or length has perhaps the biggest effect on the design. The longer the railing, the more uprights it will contain. The more uprights the more possibilities there is in design. But you need to know how many uprights there are in your railings before you can even start to work on a design.
Most of these pre manufacture parts are design to work with a 100mm gap between upright bars, the scrolls most certainly. There is something you should perhaps read before we go any further.
In an ideal world a centre upright bars gives you the best possible option for making a nice design. If you have no intention of using S scrolls then the variable spacing option is the way to go. A centre bar can be obtained but adjusting the bar spacings until we get one.
If you do want S scrolls as part of the design, then hybrid spacing might work.