Building on Intermediate -> Advanced gameplay

Okay folks,

So, I've been practising Fast-Castling for the past few weeks, but I had a few questions which some of the advanced players might be able to help me with.

1) At this moment, the only strategy I really know is Fast Castle. I do it with every civ, on every map. This probably isn't good, as it's predictable... but what situations or maps should dictate a change in strategy... like doing an Archer or Scout rush? I haven't practised these other strats, as they are different build orders, and I've only just started to get my timings good on FC, so I don't really want to confuse myself with other playstyles yet.

2) When should you drop your 2nd, 3rd and 4th TC's? I usually drop my second when I have enough wood stockpiled and I have a good location close to wood and gold. TC's are expensive and take a while to go up, so the more vills building the TC, are vills not supplying me with gold to keep pumping out Knights.

3) When the gold mines run out, and the Trash Wars begin, What are the counters to mass halberdiers and elite skirmishers? I would have thought lots of onagers, but those take a while to train, and they cost gold.

4) I tend to tunnel-vision until Castle Age when I'm Fast-castling into Knights. After that... my concentration tends to get a bit scattered after the first attack or whatever, and out of instinct, I then turtle. Are there any tips or tricks to keeping an eye on the vill production and boom at home while busy Knight-raiding?

5) Are Rams really all that useful? I've found that my late-game siege force consists of Trebs, Bombard Cannons, or Onagers. Rams are slow, and tend to get cut to pieces by anything with a melee weapon. So while using them as a portable Mantlet for Teutonic Knights may seem good in theory, they tend to get wrecked by enemy onagers and bombard cannons very quickly.

6) What is the counter to Mass onagers? Usually when one player gets a force of 14+ onagers with Chemistry and siege engineers, they have a mobile death turret that wipes out anything that gets close to it. Even paladins can't stand against it. My first guess might be lots of bombard cannons, but some civs don't get those. What then?

7) On large maps, or ones with 200+ pop games... I've heard many tutorials say that the sooner you can have 100+ vills, the better. Okay, but what are the resource balances between those vills. 30 on food, 30 on wood, 30 on gold, and 10 on stone? 100 vills is a lot to manage, and the majority of the games I play against the AI's I barely get above 80 vills, as when I get that many, then I've already pretty much exhausted the gold mines, so having an absurd amount of woodcutters and farmers seems only so useful... especially when market prices have bottomed out at 17 gold per 100 wood or food.

8) Attack or Armour upgrades at the Blacksmith first? I usually get the armour ones first, as they boost 2 stats (melee and pierce armour) instead of just melee damage.

9) Scorpions seem very ineffective. Is there an advantage to using mass scorps instead of mass onagers? They have less damage and even less range... and less HP, too - so what good are they?