Having mixed breed dogs rather than pure breds.

Hybrid vigour or Heterosis. Calves with heterosis often excel at the feedlot in terms of weight gain and carcass quality than other calves do if they're purebred Simmental or purebred Angus. This is because they have the genetic qualities of either breed to make one superior calf.

Yes, since this will really put the emphasis on heterosis for MG-Galloway X calves. Belted Galloways are relatively the same size as MG's are, so the calves will come out nice and small, but VERY growthy. Heterosis, by the way, is achieved by breeding one breed of bovine with an unrelated different breed of bovine to get a calf that is superior (genotypically and phenotypically) to either of its parents.

You can use a Simmental (white-faced one) or a Hereford. Hereford over Brangus would probably give you the best in terms of heterosis.