If asteroids were available earlier in the tech tree it would help with expansion especially for the Minbari and with the unimproved penalty would serve as an economic brake. Thoughts?
Balancing SotYR's races is always a challenge. The younger (weaker but adaptable) races, like EA, Narn, should probably have early access to all planet types. At most they would require tier 1 or 2 techs.
It would be nice if the Centauri ships had a bit of a buff (so that they were superior to early/mid level EA & Narn) but were then hindered a little by requiring a few colonization techs, say tier 1 to 3 for the more hostile environments - I couldn't see Centauri Nobility wanting to scratch out a living on volcanic or ice worlds.
Minbari do seem to need some help in expanding, so allowing them to colonize asteroids early on sounds like a sensible step. I'd keep the ice colonization tech a tier 1, but maybe move Star Bourne down from tier-3 to 2. I'd leave the colonization of volcanics where it is.
Just a reminder - all DLC planets can be colonized with out research at the moment.
I'm still unpacking boxes
You'll probably be doing that for while yet. Now where did I put that ...?