I find the best thing to do to get a REALLY good start is to first purchase 2 factories on every new planet and build the rest with the factories. Then on the first 2 planets build farms, markets, and an entertainment center. Give each planet 1 entertainment to keep moral controllable, except the economy planet which usually requires 2 or 3. Also sell "useless" technologies for big bucks! I like to save up, and then convert almost everything to research, spending more money then i make for small periods of time. Using sold technologies to power my researching power.
I like to focus on 3 main "types" of planets.
First is the Economy Planet "i find 30% of your planets to be good amount" which is balanced mix of Farms and Markets, with however many entertainment centers you need to keep good moral. With your best having economic capital. Your first 2 planets should be of this type.
Second is the Research Planet "I find 60% to be good amount", Research is your key to domination, not Economy or production. A couple Large "or better yet Huge" ship equiped with at least moderate weapons can easily mop up a much larger force of weak ships. Which lets you conquer thier Planets "with superior soldiers of course", and in the end you grow. Not to mention soil enrichment and terraforming will make the planets you do have that much better. "with your best planet having the research capital"
Finally is the Production Planet "final 10%". Build nothing but factories, and with only 2-3 production planets 1 being with a manufacturing capital the capital will give you 50% more production in total. Although you won't be able to pump out as many Ships as your enemies, a handfull of super-advanced ships can handle entire fleets. Also it's a good idea to place a couple major galactic wonders on these planets because of the abillity to build them relatively fast due to much production.
Using this strategy, Once i declared war on one guy only to find that the "alliance tech" i had sold them caused them to ally with EVERY major race in the game. 6 "building 3 more during war" of my Obliterator Class ships destroyed every military vessle the enemies could field. Not 1 ship was lost. After taking half the game's planets I then "with my superior diplomacy as a result of my research, and them being VERRY desperate" gave them offers for peace with extremely high price tags, often stealing EVERY penny most of the enemies had. Also selling technologies for high price tags tends to make them poor so they build that much slower.
