Lol, Sofi is normal. In Spanish, Sofi is to Sofía what Sophie is to Sophia although I think in English people tend to use Sophie as the original name and not so much of a nickname.
Based on the name Sofia, I've heard and seen Sof/Soff and So also been used, people have also changed the S to a Ch which changes the pronunciation like: Chofi, Chofa, Chof, Cho.
Source: my name is Sofia and have shared instances where the majority of women in a group are called the same and we have to somehow identify each