The reason there aren't any strategy guides around is because there are many ways to win the game. GalCiv II is more of a 4X than other games in its genre and that alone gives it a wide spectrum of approaches.
This is so very true in more ways than one. The easiest way to categorize the game is by the number of planets and opponents, however besides number of planets the actual physical size of the galaxy matters as well. In any case based on few or many planets in the galaxy and few or many opponents you will find that you will have to play four basically different types of games. What will work in one will probably not work in another. Within any one type of game there are many different ways to play that particular game that can vary widely. Add to this the very wide range of difficulties and the when, what, why, where and how of pretty much everything changes.
The AI absolutely can be rushed but it's certainly something that's most easily accomplished at lower levels of difficulty on smaller maps with fewer opponents. In general at higher levels of difficulty you are always going to be the weak sister in the galaxy. You'll be out-produced, out-economied (is there such a word), out-researched, out-colonized and basically out-everything there possibly is.
One way to win is to research very selectively and pick your way through the tech tree to gain some kind of advantage over the AI in some limited area that you can exploit to conquer one of the weaker AI's. From there you're a bit stronger and can move on to the next AI and so on.
Another method is to try to stay out of the way of the big boys in the galaxy and pick off a planet here and a resource there from the losing side of the inevitable wars again gradually building yourself up until you have some kind of legitimate chance in a straight up confrontation with an enemy.
There are factory focused strategies, research focused strategies, income focused strategies and diplomatic focused strategies and all combinations thereof as well as a few other focused strategies that just haven't been thought of yet.
The colony rush is certainly a very critical time. At that point no one can conquer anyone else because no one has plantary invasion so getting as many planets and resources as you can is critical. However you can easily colonize too many planets and be too weak to defend yourself once the colony rush ends. On the flip side you can focus your colony rush on quality versus quantity and develop fewer but stronger planets. If you do this however you need to be ready to jump on a weaker opponent before they have a chance to develop their larger resource base or you will be toast.
As far as planet specialization I usually do specialize planets but I have no fixed ratio of types it's more fluid and changes over the course of any one game. In general it's pretty difficult to specialize planets when you don't have many of them. If you're talking about games where you end up with 6-8 planets in the colony rush that's far too few for specialized planets. Certainly you'll have planets that will be better at one thing versus another but you'll really need elements of all three things; production, research and income on all of your planets. True planet specialization really comes into play once you rush perhaps about 50 planets but even then only really comes into play after the colony rush in your decision of how to deal with conquered planets.
Anyway, you really simply have to play and listen to people and ask detailed specific questions and play some more and keep doing that for awhile and sooner or later you'll develop your own style and find the things that work for you vesus the things that work for other folks. The only thing that I can gaurantee is that these two sets of things will be different.
Sorry that there aren't better concrete answers to give you but if you could simply dissect the game and come up with the "correct" strategy then the game wouldn't stay as interesting and challenging as it has for as long as it has.
