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.
Yes, in the Nether Zombie Pigmen spawn on the netherrack but I don't think they spawn on Netherrack in the Overworld.
Yes, but the Pigmen and Ghasts won't spawn.
Slimes cannot spawn in the Nether in Minecraft. They are exclusive to the Overworld, where they spawn in specific biomes and under certain conditions, primarily in loaded chunks below Y-level 40. In the Nether, you would typically find different mobs and creatures, but slimes are not among them.
It's possible with a InvEditor or a MapEditor (cheats)
No, mobs will not spawn in water or ontop of half slabs.
yes if they set spawn in the nether, but not many do
Chickens, cows, sheep, wolves, and pigs will spawn on well lit grass in the Overworld. (Not the nether or the end.) Mooshrooms will spawn on Mycelium in the Overworld, Skeletons, creepers, spiders, bats, and zombies spawn in dark places. (Only in the Overworld.) Zombie pigmen and ghasts spawn randomly in the nether. Skeletons, and wither skeletons spawn in nether fortresses, and blazes spawn only near blaze spawners. (Found in nether fortresses.) Ocelots spawn in jungles, and squids spawn underwater in the Overworld. Endermen spawn rarely in dark places in the Overworld, or witha much higher frequency in the end. Cave spiders spawn from spawners found in abondoned mineshafts. Villagers spawn when the map is created and when other villagers breed and have babies. Slimes spawn at low elevations in slime chunks or in swamps. Silverfish spawn from their spawners in one of the worlds three(or less) strongholds. Witches spawn in witch huts when the map is created. One ender dragon spawns in the end when the map is generated. Those are all the mobs which are created naturally, for more information, I advise you go tohttp://www.minecraftwiki.net/wiki/MobsIf all these conditions are fulfilled, and no mobs are spawning, I suggest you press Esc and change your difficulty to easy, medium, or hard, as no monsters spawn while you are in peaceful mode.In creative mode you can also use Spawner Eggs (found in your inventory) to spawn mobs
Passive mobs need light to spawn, and they only spawn on grass. If the aggressive mobs are spawning, then it must be night and the passive mobs will only spawn near torches, pumpkins, glowstone, etc. However, Notch MAY have fixed this in 1.6, but I'm not sure.
Wither skeletons will only spawn in the Nether Fortresses. The Wither Skeletons are rare mobs that are only in the Nether, more specifically the Nether Fortress. Unlike regular skeletons the wither skeleton carries a sword and when hit by a Wither Skeleton you will be come infected by the wither effect.
In Minecraft Classic, mobs can't spawn, so they can't exist naturally. If you play the Survival Test, and when mobs spawn there, you can save it and open it in creative mode so then the mobs can stay there. However, in Minecraft Creative Mode, mobs can spawn, but they can't hurt you because you have no healthbar.
If you mean by mob egg-type things, you can't. Only darkness can cause mobs to spawn.