Agreed, FreeNAS won't work. That is the whole problem with anything enterprise raid, they expect you to have almost every disk you want to have in they system day one all the same size because hell a company can afford that. Oh and you also need to afford that expensive ECC RAM. If you are a consumer who is piecemealing you need something other than an enterprise solution.
Your setup is screaming for Unraid. I have used it for 5 years on two systems and can recommend it wholeheartedly.