The beaks change based on the birds I use for inspiration. Gilda and the other griffons shown in show are very standard. Eagle and lion, which does go with standard myth, but what's the fun in that?

In the show we never see them eating meat, only standard apples, flowers, other vegetation. Why base them only on predatory birds when they don't even eat meat?

I used birds that fit their personalities. Take Pinkie for example. I used the Kookaburra, which is famous for it's laughter. Despite it's difference in beak, the kookaburra is a type of kingfisher, which is famed for it's fishing skills. I also based her off a blue-footed booby, which is one of the silliest birds just by standard knowledge, thus blue and webbed feet.

Fluttershy is based off a nightingale, known for the most beautiful song in the bird world, and the heron, known for it's grace.

They all have basis and reasoning behind their differences.