Custom grass / Tree Mass placement Question

OK I know you can mass place trees on a terrain in unity but I don't know how much that affects performance? But what I'm also wondering is if it's possible to mass place a custom Tree or in my case a set of medium poly grass model (mesh) at 175 KB FBX file? I don't want to place a huge amount of this type of grass but a few sprinkles of it here and there would be great. otherwise I could just place a few here and there.

Does anyone know more about this sort of mass terrain object placement that could shed a little more light on the subject for me?