I would contact the organizers, ask the distance between poles. They should be able to tell you distances, or refer you to the Rulebook they are using for their distances in classes. Reading the Rulebook can give a LOT of information so you are correct in your equipment, clothing, extra supplies for various kinds of classes. They might show you types of obstacles that are used in classes to go thru or over, so you can practice that as well.
I have not ever been in a driven pole bending class called slalom, so my advice probably would be incorrect information. I think the 4-H RIDDEN distance in Pole Bending, is only 21ft, which would be really tight for driving Maybe impossible for big horses and a carriage without a turnover or walking the poles! I know our big horse, 16+H was good ridden in Poles, but not nearly as fast as small horses could be. He had to be trained differently in style because of his body length, to get thru without knocking poles down.
Best thing to do is ask, then you will get the correct answer or a place to find the correct answer. Maybe the show advertising tells you what Rules they will be using, most of our Driving Shows do that.