lilpeaches,
Perlite or vermiculite will be a better option in my opinion. I have not had success with styrofoam added to the mix, it will float to the surface and unless that is what you want it will not lighten the whole piece as intended with perlite and vermiculite.
Perlite gives a white fleck on the surface of tufa if you brush it, the texture or feel of the surface is 'scratchy'. vermiculite gives a flecked mirror/mica appearance and has a 'spongier' texture feel even after you roughen the surface with a brush. perlite in my experience will give a lighter piece when dry and in my experience also not as durable.
Happy casting,
dena