1. Fleet building is typically done for pre-invasion (emptying AI's planet orbits) and to guard your transports.
The advantage of a fleet is that all your ships will attack at the same time, making for a more concentrated offense. Sending one ship after the other against an enemy fleet will result in all enemy ships firing at 1 ship of yours, then the same happens to the second ship when you send that one, etc.

2. What to build in a fleet? For pre-invasion, anything with big offensive capabilities will do. To guard your transports, you might want to have a good sensor range, decent speed to keep up with the transports and either full-offense or add some defenses specifically designed against the AI you're going to invade.

3. Where to put them? If you're only going to use them for (pre-)invasion, their place will be wherever you're going to attack. However don't park them in enemy AI space as that will warn the AI of your plans. Build them in your own space, then send the finished fleet straight to its target.

4. Military being underrated: consider researching the diplomatic (yellow) Majesty branch and building a Spin Control Center on 1 world whose orbit you'll then fill with the best ships you can build. The SCC multiplies the military rating of all ships in that orbit by a factor 4 I believe.
Also, don't fall behind too much in researching weapons and some miniaturization. Bigger hull sizes also help of course, I would advise at least going to medium hulls very early. Those ships can still be upgraded with better weapons (though upgrading is costly) whereas small hulls with good weaponry might not be enough to hold off the AI.

5. Sector defense fleets: I know several people park a few fleets close to their borders if an enemy AI is close to going to war.

6. I would go for a single tech branch: otherwise you'll end up with mediocre lasers and mediocre mass drivers instead of a powerful tech from just 1 branch. However consider that the AI will build counter defenses when at war with you. Simply researching the most devastating offense you can get and putting that into medium/large/huge hulls should do the trick though.

Always build something - unless you can use the unspent military production as an addition to social production (e.g. when building galactic achievements or other time-consuming buildings).

And you can always use either more military ships or constructors to improve starbases, so yes there's always a good reason to keep building

I am quite new to the game so I don't pretend to be an expert, but when it comes to a defensive fleets I have started to experiment with diffrent configurations.

The most succesfull was a "interceptor" class combined with a "observer" class. Both are very fast small hulls, the interceptors are parked on planets and have higher attack value to take out transports, while the observes are parolling around and have a lot of sensors.

This system is designed to take care of those damned transports that at times seem to "materialize" 3 parsecs away from your undefended planets. However, as soon as a large unfriendly fleet aproaches you have to move these light ships out and bring in your own combat fleets to deal with it.

There are two reasons I use logistics and fleets:

1) To gather invasion transports.

I have the impression sides take turns in ground combat so the more hapless grunt you drop, the harder they strike and the less damage they receive.

2) When the enemy uses bigger ships than I have.

In an effort to avoid Mutually Assured Destruction (MAD) with the introduction of ships firing back (except in the case of the Arcean super ability), the ship with the largest base value of HP (not the residual value) gets to live with 1 HP, even if it entered combat with just 1HP left. When you're facing larger ships with a high weapons rating, these ships will be pretty much invulnerable unless you fleet your vessels. Obviously, such an approach can be quite costly in terms of lost ships so you're better off making sure you have the biggest ships with the biggest honking guns.

I usually build a quite few of those huge invulnerable hulls and they are the only deep space combat vessels I bother building. I build defenders to block transports and that's pretty much it.

The approach of a lure using a fleet module is interesting but not an approach I'm using.


That said, the carrier-approach does have it's benefits:

1) Engines are costly and big. Speed modules can really shine here though I don't know how many races have that technology.
2) Fleet attack modules relieve the pressure to develop costly weapon technologies.
3) Since a fleet consists of (mainly) cheap and small attack vessels, several planets can easily knock together a working fleet as opposed to each planet having to build one huge ship. You'll have something to swing with sooner.


It also has some drawbacks compared to the all-attack approach:

1) You need to divert research in defensive technologies (at least enough to withstand the attack of the entire opposing fleet).
2) You need to research the correct type of defensive technologies or your efforts are square-rooted.
3) Since the AI doesn't seem to bother with more than a token defence anymore, even matching defences compared to your weapons won't make much of a difference.
4) You're matching your defence value to the opponent's attack value. I find it easier to match your attack value to his HP.

I'm not sure what the price-tag is is a the two approaches so I'm not putting it at pro or con.