I know Spartan-IIIs are rare, based primarily on the fact that the vast majority of them died in suicide missions, but I know some did survive. Headhunters were meant to be the most deadly of the IIIs, so the only reason to disallow it would be if we considered them too strong. But, we're deciding the power scale of our team right now, so that's not an issue. We might want to stay willing to change things around, in case we get new ideas from new players, but right now, I think everything you've suggested is fine. 

Right now, we seem to have three players, including myself, who are interested. By my standards, that is perfectly fine for a small group, but I do want to give some more time to see if there is more interest.