Three capital ships is normal by the 40-minute mark, but a mere 30 frigates is way way way too few.  I'd say you should have at least 40 assailants by this point, not even counting other unit types.  I think your big problem was that your fleet was too small.

Marzas are pretty tough, but by this point in the game you should be able to throw enough missiles at them to keep the enemy on his toes.  The Marza is a close-range capital ship that must enter your threat range to use its abilities (similar to the Kortul) and as a result if you have enough firepower you can basically relegate it to a glorified planet bomber.

As for economy, it's a matter of what you need and what you have.  Often times I find the problem isn't the economy at all, but rather that a player isn't conserving his forces or spending smartly.  It's very much about getting maximum bang your buck, and that means preventing casualties and picking your technologies and upgrades wisely.  Trade ports work very well, but as GaoFan says you want to be careful since they're expensive and vulnerable.

That's an extremely weak army there with almost no firepower.  You shouldn't have lost a single unit unless he got reinforcements, and certainly should not have lost a capital ship.  You must always retreat capital ships well before they reach critical damage so you can repair them and conserve them. 

Remember that missile defense platforms have limited range, and you can just pot shot them safely without ever getting attacked.  You're making a huge mistake if you get into missile turret range without good reason.  His overseers would give him strong healing capabilities, but if you have enough bombers or assailants you should be able to wear them down.

This can be a problem for bigger empires.  The best way to deal with these guys is to build a couple of turrets, a hanger, and a repair platform in a little circle on every planet.  This will clean up those small rebel forces (pirates, as mentioned, are another matter entirely).  As Vasari, you have it even easier; set up regular phase stabilizers and just keep a small force on standby to respond.  This can be a blessing in disguise, since capital ships will level up off of these rebels if they kill them, enabling you to cheaply raise new capital ships in the late game.

Personally, I don't research this upgrade as TEC because any empire large enough to be challenged by it has high enough income to set up a strong defense, and any empire too small to be challenged by it will actually benefit from the additional capital ship XP.  I can understanding how it challenges a new player, but once you learn to beat it, the rebels are purely an annoyance and not at all a threat.

Thought you meant Skirmisher; yeah, the Skirantra will present a more serious threat.  Against that he has some mobile damage output, and you'll need to step up your counter-measures appropriately.  With a thicket of missile platforms to hide behind and a big chunk of flak, you'll need to very carefully use your strike craft to break through.

You could try an attrition tactic, to use your own flaks and fighters defensively and slowly whittle your way through his defenses with long-range assailants.  Or, you could try shock tactics.  Flaks won't kill fighters and bombers (especially bombers) on a single pass, so you can get off some damage, and retreat to your own Skirantra to heal up. 

In any case, getting into turret range is definitely the wrong answer here.

Enforcers are hardly destroyed by Defense Platforms, I tried today) also, 2 more questions:

Map is small and pre-made: Balance of Power, 3 humans, FFA, I play as TEC

1) Do I have to try to rush enemy positions through flanks or do smth else if I have the following desert neutrals: 6 LF 4 LRM 4 FLAKS 4 KODIES 3 KROSOV while enemy (one of, other has nearly the same as me) has 1/1/1/1/1?

2)How to fight early Destra spam? I and third vasari player have built a fleet or LRF to clear desert, and at the point we colonised these planets this Adventist already had two flagships because of easy good planets and 6 Neutral EXTR. and began to build HC. What to do with that if you have BASICALLY and POTENTIALLY weaker eco. SB - not answer.

Thanks.

Yes, but enforcers are expensive and don't pack nearly as much punch (on a per-cost basis) as an assailant, plus they're weak to bombers.  They work very well in smaller engagements due to their toughness, and also will do well against a LRF-heavy opponent, but avoid them against someone whose has a large number of bombers.

I just checked out the map in question.  To put it bluntly, it's a poorly designed map; I wouldn't play it personally.  First of all, hope that the uncolonizable gravity well next to you has plenty of extractors; if you have few or none (one person has a magnetic, which always has none) you're kinda screwed, whereas if you have three it's a major boon.  You should be building military units like mad and attacking your enemies voraciously.  Don't bother with the ice planets; if you waste money on civic labs you will probably get killed.  Defend yourself using an array of repair platforms and a couple turrets to back up your fleet.

The other problem is the desert.  Your economic prospects are entirely based on that one desert planet, and you can get completely screwed over by the militia count.  In fact, the way the map's designed there's a good 90% chance that at least one player is going to get screwed over in some fashion, and I'd say a 50% chance that one player is going to be at a significant advantage over both opponents.  This is why I call it a bad map.

That kind of militia is well outside of your early-game take-down potential, especially with vultures (read: human opponents) nearby ready to steal your desert planet once you've painstakingly beaten down the militia.  You definitely need to do something underhanded, but the map layout isn't very helpful in that regard.  Going civic labs to capture the ice planet will screw you over military-wise, and with only one planet (not even an asteroid to complement it) you won't be able to maintain a competitive military.  I'd probably still try to take the desert, simply because the map gives you few counter-options.

Uh... destras require 5 labs, and probably a couple logistics planetary upgrades because there are so few planets on which place them.  Anyone who goes for early destras early is going to cripple their fleet to afford the research.  Just attack them before the destras come out and you should be able to knock out the labs before the destras come online, probably completely knock the guy out of the game.  On a map this small, fast destras should be suicide.

If by chance he does get them out, and you don't have any other advantages to counter them, you're probably screwed.

Six neutral extractors?  How'd he get six?  This implies that he controlled all the neutral extractors on the map, including those right next to an enemy homeworld!  I could believe a Vasari player was capable of pulling this off with his speedy scouts, but an Advent player achieving this with slow and expensive missionaries means someone really dropped the ball. 

You needed to attack before he got to this stage.  From the sounds of things, this is no longer an early-game situation, the Advent player controls more than half of the map's wealth, has reached a high tech level, and has a bigger fleet.  That's game; it's time to surrender.