In fleets, supposedly the combined stats are used

Where did you hear that?

Combat is played out on a ship by ship basis.

Turn 1, single ship uses 100 attack, and of course only attacks one ship in the fleet. Does it roll that 100 attack against the 50 defense of the fleet to determine damage to 1 ship? Or against the 5 defense of the individual ship?

Against the 5 defense of the individual ship it is attacking.

On the fleets turn, each ship takes a shot at the single ship. Does the fleet use 100 attack roll for each ship that attacks? Or does each ship use a 10 attack roll against the single ships 50 defense?

10 attack vs 50 defence.

But only taking into account each ships individual attack stat per shot would seem to minimize the point of a fleet.

You've already hit on the advantage in your post. You can only fire as many shots per round as you have ships in your fleet. Run your example above assuming neither fleet had any defence...that 100 attack can only kill one ship per round. Round 1, it destroys a ship, the fleet fires back and dishes out 50 points of damage. Round 2, the 100 attack takes out another ship, but the fleet probably finishes it off the same turn. The fleet's going to win every time.

That's pretty much how it goes - if the large ship can hold enough defence to more than cover the individual fleet ships attacks and still have a reasonable attack rating, it's virtually invulnerable...and this is probably the best way to take advantage of a large tech lead. If the attack strengths are higher than the defences, the fleet usually wins out. In the late game with the very best weapons, the balance swings back toward the fleet again because even with loads of defence, some shots will get through on a bad roll of the dice. If the attack ratings are high enough (late game they'll usually be at least 50 on a ship), the amount of damage that gets through can be significant compared to the large ship's hitpoints.

Don't forget that even the smallest attack has a chance to hit the strongest defense. This is because the rolls are 0-Atk vs 0-Def. So even a 1 atk rating can do damage if the defender rolls a zero.

Given the above example of a fleet battle involving a 100/50 battleship vs ten 10/5 fighters, the fighters can do some damage just due to randomness. Each fighter has a 9.8% chance of a damaging hit with an average damage of 4 (mode of 1 damage, 18.2% of all nonzero hits).

We also have to take into account the chance of a battleship attack not killing a fighter in one shot (8.42%), which would extend the fight by an additional round.

So assuming average damage, we have a minimum 10 round battle (since a battleship can only target one fighter per turn). Each round the fighters do less and less damage, as they are whittled away, but on average they should do about 21-24 damage to the battleship (depending on rounding).

This is much less than the 60 damage the fighters took, but non-trivial.

This is why you hould escort your battleships with similar throw-away ships

There's a formula to determine attack order. I can't remember it though (sorry). Something like weapons/(hitpoints+defence). Attack priority is highest value first.

Try using small or medium ships as support instead and emphasise weapons more than defence on these ships. Alternatively, if you're building defence on your tiny support ships, take it off!

Apparently the documentation must be incorrect. As the original post explained it does say the fleet combat values will be combined (and hence implied lots of small ships are better than a single large ship).

So a smart strategy then would be to have a couple (or maybe one) large ship that has the highest attack value but also very high defenses, supported by a lot of smaller ships with no defenses and high (but not as high as big ship) attack values. Is that correct?

It really all depends on your goals, and more importantly the make up of the enemy fleet. If you happen to have a tech lead sufficient to build a fleet of ships and have almost all of them survive (I find it easy to suffer zero losses up to masochistic level), you'd be pretty daft to pursue that suggested strategy since you *will* lose several of those completely undefended ships.

Let's say you're up against a bunch of heavily armed but lightly defended huge ships...it's quite likely (if techs are reasonably evenly matched) that you'll do more damage for less cost if you use smaller well armed ships with no defence. Your fleet's "defence" is then the fact you've got way more ships and most of the enemies high attack is being wasted each turn (50 attack can only destroy a 12 hp ship once!)

Reverse it, though, and heavily defended, lightly armed large opponents and your many small ship strategy is going to be in big trouble as you won't be able to overcome their defences. This time, you'll have to go for larger hulls which can fit more offensive power.

If the enemy has a mix of hull sizes and ships, your best counter is also likely to be a mix.

As you can see, there's no "best" strategy and hard and fast rules. It's all a balancing act.

Personally I don't bother mixing ship types much beyond tailoring the form of defence and attack according to the opponent, but that's because I usually have a strong tech lead.