I think those with RSD may be social avoiders but may not be. RSD is the reaction to things said or done by others, which could cause social avoidance but they aren’t the same thing.
I don’t know that there’s any difference between hyper-fixate and hyper-focus other than terminology.
Here’s more on both topics:
Penny
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism