they are not very compatible

I think Snow White would be all chirpy and friendly to Merida, trying to get along with everybody. But Merida would kind of avoid her.

I think they would definitely be good friends though not greatest of friends. I think Merida would find Snow to be a bit optimistic and upbeat but Merida is pretty energetic and upbeat herself. I think Snow would be the friend that Merida went to when she wanted to understand her own mother a little better and vent about her problems with her mom. I think Snow would admire Merida's bravery and courage but not fully understand it. I also think Merida would love to eat Snow's bakery treats (i.e. pies and cookies etc.) which they could talk over. Although Merida might find Snow to be a little too demure and Snow find Merida to be a bit too abrasive, I think they would get along most of the time and not have much to fight over.

sure, why not? they like animals and the outdoors and speak their minds. Even though she wouldn't like how wild Merida is she would tolerate it because that's the kind of person Snow is, and Merida would appreciate it. I think she'd definitely be her favorite of the classics, even if that isn't saying much. The other two, however, no. Merida wouldn't understand Cinderella at all but not really dislike her, and Aurora she wouldn't care for either, and maybe even feel jealous of her beauty and effortless queenliness.