Here are a few resources that might be helpful to you:

https://forums.galciv2.com/104908

https://forums.galciv2.com/151466

How fast to expand?  -  In most games agressive expansion early is rewarded.  The more you expand, the more resources you get, and the more weight you have behind you.   GalCiv2 is one of thoes games.

Turtleing at your base means you only have 1 (2 if your lucky) planets that are giving you money and tec.  More planets early mean more money later.  If you dont do it, the AI will, and they will use thoes extra resources to take down your slowly developing planet later.

fleet composition? - I've found that a swarm of 7 tiny ships with 2 lazers2 each are a nice deturent.  They work well for attacking, and they have enough of a punch early game to bite into anything coming your way.  They also make good attackers.  They are cheep enough that you can pump them out while doing other things and keep any losses replaced. Of course this only works early game.. later on more dangerous things are out there.

Personaly I prefere to attack them before they attack me.  The derengi want nothing to do with me if I have a swarm of attack ships out ripping the Yor Collective to bits so I can expand into my planets that the they are squatting on.

Your economy is very important so you'll want to research economic techs early. Morale is important too. Happy people allow a higher tax rate. I also make it a point to research sensors quickly so I can get a couple of survey ships out to help my flagship reap annomalities.

I go with an "all labs" strategy which basicly means I build no factories and use "focus" to build social projects or ships. I start off by rush building a lab then normal build a morale structure and an economic structure. I fiddle with the spending sliders pretty much every turn for a long time to keep morale at 100% (for faster population growth) and my tax rate as high as it can be for that max morale. Sinc I use "all labs" my research spending is kept at 100%.

Whatever strategy you use you will want to devote some planets to economy. Build enough production buildings so you can build at a decent rate and then build only economic buildings on the planet. Only build starports on planets that you intend to produce shipts at.

If you establish trade routes with other races they are less likely to attack you which means you get to chose when to go to war. And don't forget to bribe other civilizations to fight each other. The more they fighte each other the less attention is focussed on you. If 2 civs have missiles as their main weapons and are fighting each other you can trade ECM tech to both civs to help keep them in a protracted war. Or if on is significantly stronger just trade ECM to the weaker civ.