Hmm, was it a pirate fleet that attacked you? They don't have capital ships but they can have some nasty cruisers. While you are learning the game I'd play with them off.

Its really hard to say if that's unusual or not without knowing how long that game was. After the first hour yes that is certainly possible. Slow expansion and over researching is a common problem for new players, so that is quite likely your problem.

Hard AI and above do cheat. I'm afraid Easy AI is either the same or has a resource penalty compared to you. Also I don't think culture has anything to do with the AI attack patterns.

If you'd like to get a feel for Sins without that "buzzkill", you can play a map without any AI players at all. That way you can get some practice colonizing and learning what each tech/research does before fighting for real. If you want to get a bit of military experience too you can leave the pirates on (just remember to turn them off again when playing for real), they may win a battle but they won't attack more than one planet per raid, and you always know when they are coming.

Pirates are balanced against veteran players; I'd advise keeping them off until you can beat at least the hard difficulty.  Beyond that, the key is to keep pacing and find the right balance of fleet size, expansion, economic investment, and technology.  Most beginners tend to over-emphasize tech and economics, and under-emphasize fleet size.

A good way to get a feel for early-game combat is to load up a small map (Close Encounters would be perfect) and try executing a basic rush strategy.  As TEC, build two military labs and research the "Javelis LRM" unit.  Choose the "Sova Carrier" as your first capital ship.  Demolish your capital shipyard once you finish building your Sova and replace it with a second frigate factory.  Capture the nearby asteroid and ignore the larger planets, and launch a direct offensive against the enemy once you have about 20 frigates.  Keep a constant stream of reinforcements heading towards the front lines.  This should give you a good taste of combat and how quickly you can field units.  Although a human player will beat this if you don't diversify with other unit types, the lower difficulty AI's will just get crushed by this efficient rush.