First here is the wiki: http://galciv3.gamepedia.com/Official_Galactic_Civilizations_III_Wiki
Give me some time to write the rest of the answers...

The weapons and defenses have 3 categorires: Missiles, Beams and Kinetics

Missiels have the longest range, slowest fire rate, highest damage.
Beams have medium range, medium fire rate, medium damage.
Kinetics have lowest range, highest fire rate and lowest damage.
For more detailed stat go into the ship designer ingame, as the stats can also be modified by research and augmention modules on the ships.

Fleets are most relevant for combat, the amount of ships in a fleet is limited by your logistic cap which you can improve with research. Fleets combine the stats of the ships they are made up of, it has the sensor range of the ship with the highest sensor range in the fleet and the lowest movement speed and range of any ship in the fleet. So for best effect make sure the ships have the same speed and range while only one ship needs to have a good sensor range.

There are whole ship styles and races and other mods to download but not in the steam workshop as that doesn't work there yet, look on nexusmods and the modding subforum instead.

The most common tactic is to specalise planet so they can do one thing realy good. Build one or two factories first to speed up the building, as many farms as you can keep happy and the rest research/production/economy depending on what you want to do with the planet. (To improve approval build starbases as they don't take space on the planet).

No, at the moment there are no real penalties for expanding to quick(except maintaince cost) but there will be something like that in an upcoming patch, whenever it might come...

That depends on multiple things like on how many fronts you need to fight and if your fleets are strong enough to beat enemy fleets without losses and how many movement points your fleets have. If you can win without losses and your fleets have high movement point, one per front is enough else build more.

Yes, ships and buildings have a maintaince cost if you get to far into the negatives you enter a forced pay of your debts mode where all your planets change to economic focus until you get into the positive again.

Yes, if you shoot down all enemy transporters with your fleets you don't need them.(no transporters -> no invasions ->no need to waste tiles) To make sure you get them all build some ships that are filled with sensors, so you can see everything inside your empire.

Easy spent research on the diplomacy branch to get diplomacy points and build some buildings that give diplomacy point.

Trade is most usefull to buy techs you don't have and some times in the early game to buy resources so you can build ships with prototype weapons to defend yourself.

I don't have the editior so I can't help with questions about it.

Negatives:
Dead planets have some use with certain galactic events where they turn in better planets and there is more stuff planned for them in the future.

Yeah, that's one thing civ does better that galciv with the ingame advisors and stuff.

Puh, you sure ask a lot of questions in a single post.
If I missed anything important, I'm sure other people or the wiki can help out.

Don't know much about what is possible for mods, but as for the rest:

One attack type is best, because that way you only have to break through one of the targets defences.  

Kinetics in my experience, as long as you do something to boost the range and accuracy of your kinetic weapons.

The biggest reason you'd want to use a different weapon type is to avoid the best defence type of the opposing fleet.  Other than that kinetic weapons will do the least overkill, so if you're up against fighters they're probably the best, but practically speaking the biggest consideration after opposing defence is maintenance cost.  Beam maint >>> kinetic > missile.

Edit: I should also mention that the initial build cost is kinetic > beam > missile, but I don't think it's hugely important.

Black holes are space terrain and where antimatter spawns.

These games are called 4X games and there are lots of them! Check out eXplorminate.net for a bunch of others and some coverage of GalCiv3 and other Stardock 4X games!