anyone can have magic?

I think Emma always had magic in her. She just needed time to unlock it. Rumple I don't think got magic until he became the dark one. I think some people have seen what magic can do and prefer not to posses it. For Snow and Charming, I think they prefer their sword and bow and arrow. I think after battling a powerful magic Regina and reluncantly going to Rumple for his dark magic to help them, I think they prefer not to risk obtaining magic for possibly fear that magic might make them go dark as well. Maybe it'd be cool though if Snow could learn fairy magic from Blue or something. I think it's true that anyone in the story could learn magic but not everyone really wants to. Another thing is that Snow and Charming are briefly scared of Emma when she used her magic so they might not want to risk being feared as well. Course they do feel guilty about pagganap scared towards her but still they might not want the citizens to fear them if they had magic.

Rumple wasn't born with it. Zelena, Ingrid and Elsa all were.

I think that the character with magic (other than Rumple) were all born with magic potential, but they had to learn to use it. The characters without magic either don't have that potential, or didn't learn to use it/learn they had it.