I dont believe that steven will never use with another gem because Stevonnie proved that steven can fuse.

Steven may not be able to fuse with a gem because he is part human but he is only part human and he fused with connie so its like

1 part human + fully human = successful fusion

so with this theory I can assume

1 part gem + fully gem = successful gem fusion?   

Its obvious that it was the gem part of steven that caused him to be able to fuse with connie because ...From my own experience humans dont fuse....but stevens human part was also a huge part because Greg couldnt fuse with rose.

So I believe Steven has the power to fuse with Both humans and Gems