I'm actually playing one of those right now and doing pretty well. I've gained control of almost my entire starting solar system. I've been sharing some of my experiences in this thread.

My early game strategy was to just expand. I expanded quickly to about 7 planets and built them up. But I didn't expand so fast that I couldn't defend them. The last couple of planets that I colonized were key choke points where I could easily defend my epmire. Once I was done expanding I began working on my economy and making sure each planet had a trade port.

Playing diplomacy right was the key. Even if you make an alliance with someone, eventually they will brake it because it's impossible to complete every mission that they give you. Make an early ally with one of your neighbors then attack your other neighbor but keep an eye on your ally. If your allegiance with them falls below 60% you had better fortify your border because they are going to attack.

Key is to have 3-4 fleets gurading key chokepoints into your empire and only expand if you can defend your new colonies.

What I did for my planets were 15-20 turrets, 2 hangars, 2 repair platforms and a planetary shield. Use fighters in the hangars so they can take out enemy bombers attacking your defenses. This defense could hold off an attack from a few capital ships for 10 minutes at the most, but that's more than enough time for me to get a fleet there. Planetary defenses are meant to stop an attack, only to delay it for enough time to get a fleet there.