I think there is too much of an age gap between them (unless Cinderella is younger than I think)

They would be more like a mother/daughter sort of relationship, rather than best friends.

Ariel and Rapunzel have so much in common. Ariel dreamed of being human and Rapunzel dreamed of leaving the tower. They'd be great friends.

I think

Ariel would be too overzealous and risky for Rapunzel

nope.

i think ... because ariel and rapunzel in spanish i say no se pelearian mucho .

Rapunzel wouldn't like the way how ariel left her real father because she herself honored(and loved) Gothel when she thought her as mother even that Gothel was so much worse than Triton who love Ariel.
Snow White and Cinderella would be good frieds

Agree with Mongoose09 :)

I always pictured Ariel and Rapunzel as being bFFS. Cindy and Snow, not so much.