Speaking to the original topic, an important part of a good colony rush is knowing when to end it. There are often signs that will alert you - large numbers of alien transports streaking through your outer perimeter is the most obvious. There may be a few low PQ worlds left in your sphere of influence, but you will have to rush buy transports if you really need those planets. Better to cut your losses, usually, and stop building transports.
If the horde is not at your gates quite yet, but you already have enough planets to sustain your economy during the next phase buildup, you need to consider stopping your colonization anyway and switching to constructors, freighters, and warships immediately. Leave a few low PQ planets to the AI (especially extreme environment planets) on purpose. This will extend their colonization phase and delay their aggression for what could be the few vital turns it takes you to keep up with them.
If your empire is compact, your influence may overwhelm a few of these planets later. They will become a source of friction between you and the owner, but keeping up your military rating and sending them a freighter or two will keep relations calm for a good while.
Unless I am absolutely desperate to invade a neighbor early on, the months immediately after I stop colonizing form the most crucial point of the game. Building infrastructure AND military power AND keeping my research and economy going depends on not overextending myself by colonizing too many planets; and also by using every trick in the book to keep the AI factions from escalating too rapidly at the same time.
If I am forced to go to war early, of course, the real infrastructure buildup usually has to wait for that to draw toward a close. By then my economy is usually hurting and I know that my military rating is the only thing keeping the jackals at bay.
The post colonization game is a fine balancing act that begins with knowing when to cut off the colonization phase. Too few planets means not enough tax infrastructure to sustain the empire; not enough worlds dedicated to economy buildings means the same thing; and too many planets can create a spiraling maintenance cost nightmare that your scattered population can't keep up with. Especially if you're trying to build improvements and starships on all of them.
Above all, don't let my tone fool you into thinking it's easy for me. I struggle to find the best stopping point almost every time. It's not always easy to spot, and there is always the temptation to try to get just one more planet. Or go for just that one extreme colonization tech that will let you grab the radioactive PQ 19 world that's hanging right there in your grasp. Then you notice the Drengin frigates hanging around your space while you have only got small hulls armed with Laser II. Be careful out there!
