I like the meaning, and it's got good history. I used to know someone whose father was
Silvester and went by Sil - I think in their case it was a family surname.
Yup it's usable - it's had a fair amount of use here, especially on people with African last names and middle names (possibly Catholics, given that it's a popename?) I can't see it as unisex at all. especially given Stallone. But Sylvestra &
Silvestra are quite nice?
The Si-spelling appeals to me slightly more, because less popular-cultural associations.