... but I haven't taken slavery. Is it worth it? I think I took it once, was upset I couldn't whip the pyramids (lol), and never took it again...
Slavery's bonus is halved when whipping Wonders, it's mainly used for buildings and units. Slavery is incredible once you get used to using it well! As everyone says,

is king in this game. Slavery helps you turn

into

at a rate of 1 citizen to 30

, 45

with forge, etc.
A granary helps get back up to half the food needed for the next citizen, so it can sometimes be about 12-14

to 30

, doubling the value of a working citizen.
As you use it more and more you'll get used to seeing

count rise in the cities you're whipping more often, so you'll learn to try and whip when a building/unit is 31-60

from completion, so you can use 2 citizens to double whip, but only netting -1

for 10 turns.
*These are all numbers for Normal speed.
As for stacks of death, I usually do a fast expand strategy to start the game off, then after getting a World Wonder or 2 I hone in on an enemy to kill. So I warmonger and control the stacks of death myself. When I see an enemies stack, I have 3-6 catapults nearby usually and can prepare to sacrifice them into the stack before my 15-20 other units wipe it clean.
If I want to play a defensive game and sit on 7-9 cities until I have the war techs I want in the midgame (like a cuirassier rush), I'll keep enough units to defend a city, so 1-2 per city, and watch all my nearby opponents with a scouting unit in their territory. I'll make sure they stay happy with me, give them a tech if they demand it, etc. If I notice they're getting angry and have a high unit count, I'll start unit production of my own. <This is another instance where

helps sooo much!>
If I still don't want to warmonger in the midgame, I'll make allies and make sure at least 1 civ is friendly with me.