Soy milk tastes quite different from cow milk. Yet coffee chains seem to make their milk-based drinks from the same coffee pot. The pot is most often made of dark-roast blends "from the house", and it seems a good match for cow milk, as the fatty flavour balances the strength of the black drink. Dark-roast blends coffee seems overly strong for soy milk, though, and I am guessing that some cafes do serve "soy lattes" based on lighter roasts (from taste, not from observation at this point).
Blends are usually secrets in contents and proportions. The chains use the same pot anyway, but some cafes may be picking a different blend as well, more suitable to the flavour profile of soy milk.
Are there any criteria on the bean and/or roast when one intends to mix with soy milk?
Note: This question may seem subjective. The goal is to understand what concrete parameters are "available" to make a drink that preserves the coffee qualities and merges nicely with the soy milk flavour profile.