I'm pretty sure they aren't able to fuse because they are more human than gem (75% human against only 25% human). Steven is only half organic so he can fuse with a gem, but Stevonnie is too organic to fuse with a gem. They can't fuse with a human either because fusing is a gem ability and their gem part is too insignificant.
do you think Rainbow Quartz 2.0, Smoky Quartz, and Sunstone will be able to fuse with Connie?
No. Sunstone's gem part is way bigger than their human part, because they are made of two gems and a half gem. Same for Smoky and Rainbow 2.0, they are more gem than human. Connie can only fuse with a half gem, half human and the only being who is like this is Steven, so she can only fuse with him.
To be fair, it was never explained how part human a Gem has to be before they are too human to fuse. Maybe just being part Gem, whether it's 50%, 25% or even 1%, if enough for fusion.