This doesn't really sound like ICS to me - there are a very limited number of colony sites, which is the opposite situation from ICS, where you can fit a ton of cities into a relatively small space. But then again, I never liked and never used ICS in any Civ game, so I don't exactly have much experience with it. I found it rather distasteful and avoided it.
So there's a positive pressure to expand as quickly as possible. But if I understand right, there's also a negative pressure from expanding to the exclusion of defense. If you expand and someone else builds up transports and warships instead, they can take the planets you just colonized, thus robbing you of not only the planets, but also the production you spent on the colony ships, and the population you shipped off to that planet.
So, do the AI players actually hit you if you expand without defense? And do any of the AI players try to expand without sufficient defense?
Edit: Deleted two paragraphs which I decided weren't needed.