They do run the odd other brand - you may see a Mack or an International, but by far the most common are the Volvos, and they come in several different models of day cab and sleeper depending on the service.
They are so common, that perhaps they go unnoticed, but one that I saw last week certainly caught my eye. It was a company tow truck/wrecker, which apparently works out of their Moncton service shop. It was in Halifax to drop of a tractor for Newfoundland and to pick-up one from Newfoundland, that Oceanex delivered from Halifax.
A very slick looking rig, it appears to be spec'd with a slightly longer wheelbase than most of the company's tractors.
Among the divisions in Armour is Pole Star, which also generally runs Volvos with slight differences in trim.