I don't think the problem is the rapid colonisation so much as the initial range and the map sizes.
In the original GalCiv, you were pretty much limited to the neighbouring sectors until you began advancing the propulsion tech tree, even a small map size would leave around 25% or so of the map unreacheable. In GalCiv 2, on a medium map all but the far corners are reacheable from the beggining.
You could achieve a slow down simply by restricting the range further (perhaps adjacent systems only at first, growing with each propulsion advance). This would slow down the initial colony rush while still giving a fairly even start for all players. It would also make the life support modules actually worthwhile
It doesn't provide too much of an advantage to those who extend range first (although since this would place them at a superior tech level then perhaps it should) - you can use starbases instead to increase your range to compensate. It would actually give a rather interesting dilemma - concentrate on propulsion research, or use starbases and hope no-one hits them.
You can use starbases or other planets to increase this range of course, but this leads to more strategic thought when it comes to colonising planets, rather than just grabbing anything with >0 PQ. Do you colonise the PQ 11 planet in the next system, or grab that PQ 5 planet to bring the PQ 18 planet into range.
