Even if my child is poor at group activities, can they manage in a mainstream preschool or school?

Often they can, and the fitting environment differs for each child. In common: with accommodations and support (visual steps, small-group settings, a place to calm down, support staff), some children perform well in a mainstream preschool or school. On the other hand, some children grow with more security in a more supportive setting (a special-support class or school). It's not which is good or bad. What the family decides: whether to weight "the same environment as everyone" or "an environment where the child feels secure" is for the child and family to choose. A guide: check via a visit or trial whether "the child seems able to spend time securely" and "needed accommodations seem available." The environment can also be reviewed along the way.