We are starting all our horses on side reins and have a large Paso Fino to work with, but not sure if side reins would restrict their movement too much.. Can't find anything on google, help!

We are staring all the horses on them because we need to build their toplines. We are a therapeutic riding center and the specific therapy we do is very demanding of the horses. Building their topline will help them withstand the riding and thus protect both themselves and the riders.