Okay so I am going to talk about Stevonnie (Steven and Connie) now.
Every time Steven uses his healing powers, he can be able to fuse with that person he used the healing powers on. For example, Connie originally had glasses and they happened to be drinking juice at the time. Connie accidentally drank from Stevens juice box (ew but bare with me). So, with Steven's spit, Connie's eyes were healed.
Later on in the series you meet Lapis Lazuli. She was originally trapped in a mirror but then Steven released her. After Lapis realizes that Steven is a Crystal Gem she starts hating on him just because Homeworld (a place that is under the diamonds rule) doesn't like the Crystal Gems and they are against everything they believe in. Lapis out of anger takes all of Beach City's Ocean.
Later in the episode Steven realizes that Lapis's Gem is cracked. Therefore, this is the reason why she did these horrible things. Steven heals Lapis with his healing spit and her gem becomes healed. Lapis is normal now.
So really what I believe here is that whenever Steven heals someone with his healing spit then he can be able to fuse with that person. You just have to wait for the right moment. But then again Steven fused with Amethyst but he didn't heal her with spit but with words. Healing can be in different forms.
