Yes
slime? they are only in selected chunks and only spawn deep under ground.
Only when you are on easy, normal, hard. They also only spawn in light levels under 7.
If you go near them they blow up. You can kill them with a sword. They spawn only at night or in caves; Somewhere dark
The only way to spawn herobrine in minecraft is to use mods as you cannot spawn him in vanilla minecraft.
No mobs will spawn on peaceful, you may hear noises but they will be effects even on peaceful. the only entities that spawn in peaceful (not including the nether or the end) are players and animals unless, in the end or nether.
The only mob that can spawn in water is currently a squid.
You can't, they are only spawned naturally. You can, however, use a spawn egg in creative mode to spawn them.
Only is you have Mo' Creatures and (MCEdit?) installed.
Minecraft seeds only control the landscape. The creatures spawn randomly.
To find a spider, first make sure that your difficulty is not set to peaceful, then go outside during the night and soon you will find a spider. You will also likely find creepers, skeleton archers, and enderman.
The Ender Dragon and the Wither are currently the only boss monsters in minecraft.