I'm going to answer this in terms of flavor, and then acidity.
Generally, people choose more lightly roasted coffee to preserve the terroir flavors of the bean. These flavors usually come from unstable molecules that deteriorate quickly after the coffee is brewed. When coffee is brewed and consumed relatively quickly, the drinker can enjoy those flavors. Cold brew produces a more muddled mixture of flavors and relies more on mouthfeel and big flavors like sweetness, savory, earth, chocolate, etc. More delicate precise flavors are lost. It is also important to note that some fruit, vegetal, and herbal flavors are enhanced by the presence of acid, like adding lemon juice and tarragon to a fish filet.
So, if you are trying to avoid acid but you love the fruitiness of say, a medium roasted Kenyan single origin, you could cold brew it and see if you still get the flavors you want. If you hear about another light roasted coffee that has strong sweetness but is more acidic than you usually like, try it as a cold brew.
I.e. if acid is what is holding you back with specific coffees you want to try, try them as a cold brew. Just know that the lack of acid in the cold brew may make it a little lack-luster compared to a traditional cup, especially since tasting notes are made when evaluating hot brewed cup.
Darker roasted coffee as already lost more of the bean flavors and gained more roast flavors. If anything, I would say that a darker roast is going to have a more consistent flavor between hot and cold brewing simply because more of the delicate flavors have already been burned away. I would bet that Kicking Horse and Blue Bottle suggest a medium roast for cold brew because a medium roast preserves the beans own characteristics while introducing the roast flavors. This is why single origins are usually roasted to full city or less.
In summary, I would suggest that you find a coffee you like already as a French press or pour over and cold brew that regardless of roast. Lighter roasts may lose some of their complexity in a cold brew, darker roasts will mellow even more in a cold brew. As backwards as it is, I hope this helps!