Units seem to gain health as they level up, but not hero/sov types - those you get the point to spend. I'm playing Tarth too and they just get a % health boost.
I honestly have no idea then. As the tarth, by level 3 my heroes/sovs would all be at 20+ hp without putting any further points in con. This is was all prior to 1.05, and in the late beta. Unfortunately, I think I nuked all my save games, so all I can do is ramble like a crazy person.
