When Itachi said Susanoo equals combining the darkness of Tsukuyomi plus the light of Amaterasu. The literal interpretation is Amaterasu + Tsukuyomi = Susanoo, but considering that Shisui got one, I think we have to take a liberal interpretation, that is, the light of an MS tech + the darkness of an MS tech = Susanoo. In this case, Shisui would utilize the "yin" of one Kotoamatsukami and the "yang" of the other Kotoamatsukami and combine them to form Susanoo.
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I would consider part it part of cannon. For example, let's take LSSJ3 Broly. Broly never actually went SSJ3 but since he makes an appearance in the video game, I would consider him having the potential too. So if we were ever debating Broly vs Buu or something, I would assume that Broly has the potential to LSSJ3 somewhere in that battle and Kill Buu.
Same deal with Shisui. We'll probably also see some video animation in the video game with him activating Susanoo.