Welcome to the boards.
GaleRazorwind was more speedy than I to post but i'll still throw my 2 credits in.
I'm not a nid player but I face them quite often and I think I can lend some insight.
1. Short answer yes as long as a quarter of the model is covered. But read page 18 of the rulebook where it talks about the interactions of "Intervening Models." However this sort of question is usually best posed in the Rules and Questions board. No worries tho.
2. I would include him with your unit. It makes him more survivable; the other warriors can soak up hits before he dies and gives your enemy a Victory Point, because I'd shoot him first if he was by himself. I'm assuming he is your HQ. LW/BS is a good combo.
3. Poisoned works well against units with toughness values because regardless of an enemy's toughness you will always wound on no worse than the specified number.
They are good against high Toughness models but against low toughness the Warriors have a decent enough Strength to handle most infantry.
However Poisoned does beslubber all against armour because it doesn't use your strength. The poisoned approach may not be what you're looking for if you were having trouble with Dark Eldar armour (arguably the weakest in the game). The "higher" Strength of your Warriors may be better put to use attacking armour.
4. My friend has this issue against my Imperial Guard. Cover is your best friend in this regard. Try to keep as much terrain between you and their guns as you can. The Venomthrope is a good model in this regard as it gives all your units in a certain range a half decent cover save.
Run. Run. Run. Run as much as possible. If it don't shoot: Run. If it shoots but not that far: Run. It may be only a few inches here or there but after 2 turns of running you can have moved 3 turns worth of distance.
Have no illusions, you are going to lose models. Lots of models. But you put it perfectly here:
When I did though was quite funny to see 1 Warrior with 2 wounds kill 10 fire warriors before he went down.
This is the crux of it, your units are all pretty beast in CC, so much so that you don't need a boatload of them to get it done when the lines hit (but it doesn't hurt right?). I have seen 3 Hormagaunts with adrenal glands wreck a Hammerhead in 1 turn. That will ruin any Tau player's day, getting his 150+ point vehicle (they're cheaper now i know) popped by about 30 points of bugs.
In the last game I played 1 Geanstealer Broodlord (who is one of the craziest units in 40k) took out a Mortar squad a Command squad and a Lord Commissar (the latter 2 with Power Weapons) before I had to turn a 30 man squad on him and shoot him dead lest he kill my whole army.
What I'm getting at is that you don't need to get everyone there to do damage.
5. As I mentioned Dark Eldar vehicles are soft by most standards. The stock Deathspitters on your Warriors should be sufficient to destroy them. Venom Cannons are also good for this, your warriors can take them too. Alternatively you can attack it in CC. Your warriors can all glance it to death, if they take adrenal glands they would be able to penetrate its armour (due to them getting Furious Charge) and if you give said glands to your Gaunts, they too can glance them to death.
6. Raveners are a tough play. Strictly melee guys here. A amphetamine parrot tonne of attacks but they are very fast and not
synapse like you say. If you don't have any synapse that can keep up with them then it's academic. But I'd say try out just letting them loose and try to keep them on a tighter leash and see what works better for you.
Personally I'd deep strike them behind some cover and then next turn whip around a corner and ruin some poor fool's day.
Don't be sorry, these forums are here for help and discussion. Plus I ask tonnes of stupid questions all the time and I haven't had any trouble... Yet.