Why don't you get a few more of each and then sell the ones you don't want to keep once they get a bit older? That way if you lose one to disease or an accident along the way then you'll have an extra. Just a thought, I'm not a fan of raising so few chicks at once, especially chicks of different ages. Its never worked well for me.
Well, I think they'll be alright with each other. Plus, I already ordered them. My friend raised two chicks, and they did great! The feed store lady said that would probably be alright. But, thanks for the concern. :)