Doing some looking I found the 2 following book references on the matter.
- Page 71 of Savage Spirits. "If not specified otherwise in its riding mount profile, the silhouette is 2.
- Page 212 of the Edge of the Empire Core Rulebook. "The most important thing to remember is that most characters are silhouette 1.
Looking at the examples of silhouette 1 they provide, they are all humanoid species. Which lead me to believe that the humanoid adversaries are mostly silhouette 1. If it is the case that all adversaries that don't have a silhouette is 1 then there are things that are brought into question like the Carrion Spat on page 19 of Suns of Fortune, which says in description:
"Once tamed and used to both saddle and rider, carrion spats make excellent mounts, especially in rough or mountainous country where their innate sense of balance and wide-splayed feet allow them to move through. broken terrain with ease."
Yet there silhouette is not listed. That would make them a silhouette 1 and un ridable, despite the description. The key wording in savage spirits I think is "riding mount profile". I suppose that I could assume all adversaries with no silhouette would be 1 and if it has any mention or hint of being ridable in the description, up it to 2. Also I do think that having an actual place besides special abilities for silhouette would be better.

