Breastcollar is pretty common with light sleigh use. Full neck collars are hard to fit well, and have them STAY fitting well as horse changes neck in fitness levels.
Light sleigh is nothing to move for a horse used to driving, so breastcollar should work fine. V shape around windpipe will allow horse to lower head better than flat strap across chest, if going is a little hard. Good puller lowers head for better balance.
Sleighs pull and RIDE differently than carriages. Sleighs are made to FLEX and shift over the rough ground of frozen roads and tracks. If you are sure sleigh is solid, then just enjoy the "movable ride" they give. My first ride I was SURE the sleigh would fall apart around us, it moved so MUCH. Like being the candy when the wrapper is taken off!! Never had any issues with parts coming loose or breaking, that just is the way the light sleighs are made.
Other big issue is having steel on the runners that is thick enough to handle any bare ground. Old steel on runners may be quite worn. You need to check that before hitching. Not that hard to get new steel put on runners if you need it.
Question about your horse. Has it ever pulled dead weight like tires? You might want to practice with a tire a bit before hitching to the sleigh. Just a very different feel than light carts, which are so easy to move. Horse may have forgotten how to handle dead weight, so lesson beforehand, is helpful.
Then there is a "breaking the runners loose" step, before driving off in the sleigh. Sleighs and sled runners run on a layer of water. When they stop moving for long, that layer freezes runners to the ground. So giving sleigh a "shake" or sideways push before entering, breaks the frozen runners loose. If horse is reliable, they are often asked to step sideways, right, then left, which breaks the runners loose for you. Then you ask horse for forward and sleigh glides off. You DON'T want to ask new-to-sleigh horse to try pulling a frozen sleigh. Many horses don't take the resistance well!! We have seen them rear, start kicking, leap forward out of control, with the surprise of no-forward using common, light pull needed for moving a cart.
And LAST suggestion is getting traction for ice and snow on the horse. We use the tube rim pad for snow removal on shoes. Rim pad has a tube inside the inner edge of shoe. Flexes and wiggles with snow pressure, ejecting any build up on the sole. We like it better than bubble pads used in snow, because rim pads leave the sole open for cleaning. Bubble snow pads have packing under them, cover the whole sole. In our area, even with good packing, the mud and stones will work under, can make a nasty lump under the pad. We use the drive-in ice studs on the shoes for traction. They give a good grip on any ground, even sheet ice. Studs don't wear off as some other traction methods can. They are fairly small and unobtrusive, nothing like the old fashioned "sharp-shod" caulks of times past. Locally the Hunt Club uses the snow tube, rim pads, with ice caulks on their hunt horses for best footing on their runs. We call it the "hunt package" if you ask for it for your horse. The Hunt folks ride every scheduled day, 3 times a week, in every weather. With this under their hooves, there is no excuse to stay home. They cancel for sheet ice, which cuts up the hounds feet. NOT because the horses can't go!
Have fun sleighing, we really enjoyed it when we had our sleigh. Dress WARM, take a cover for your legs. Work up to bells, after horse is used to them unhitched. They do ring for a LONG distance. All our neighbors would ask if we had been out driving when we meet in town. "Oh we heard the bells on the wind" and they were between a mile to 3 miles away.