All Fusions with Steven will have Steven's shirt and shorts. His clothes don't merge because they are not a part of his physical being unlike actual gems (ex. Stevonnie).
Also, I believe that Stevonnie can fuse as well. She has a gem and her body can turn into light. Garnet said that's all you really need besides a partner you trust with that light. That also means any human could fuse with Stevonnie and then fuse with any of the gems!
VOUS LISEZ
Steven Universe Theories (Continued)
RandomThis goes along with my previous theories! I hit the limit on the other book.