Chopped fibres may reduce the chance of cracking. I'm not familiar with 404, generally only use microfibres (which are small versions of chopped fibres). I wouldn't expect it to be a problem mixing 404 with chopped fibres, hell the purpose of the additives is to join/fill/fair fibreglass products, adding fibres to it shouldn't be a problem. You won't be able to add too much choppy anyway because choppy bog gets very hard to mix after a certain amount.

Any cracking/chipping would only be on sharp edges and would probably only be a concern with regard to appearance, not the structure of the mount.