The fame loss for razing a town is equal to the fame loss of losing one to the enemy.
No fame loss for an outpost (no build que yet and such)
Otherwise you lose 1 fame for every 4 thousand units of population, rounded up. So a 6.500 population town equals two fame lost if you raze it.
After having conquered a town, you can indeed no longer raze it, just turn them into settlers. This does not result in loss of fame though.
Dark elves are the most hated of all races. If your enchanted fortress is in an dark elf city you generally get 20% extra rebels in conquered cities. This is on top off the percentage created by taxation: the two percentages are added.
Barbarians only get 10% unrest if conquered by dark elves. Dwarves and trolls, as you noticed get a massive 30% extra unrest. With default taxation that becomes 50%, or half the population. High elves are worst, getting 40% extra unrest.
But should you raze everything then? I say no. The fame is too important too lose, since every point of fame reduces your army upkeep by one, making having a large army actually viable. And dark elves lack the population growth rate to colonize the entire world in an acceptable time frame. Death magic, is not ideal in reducing unrest, so that will not help you either.
Making dwarven units is a good idea, the military units do not suffer from the racial tension. For maximum productivity, do not have dark rituals in that city, and have 8 units of garrison (normal units). The garrison will reduce unrest by 4, which you will need due to lack of advanced unrest reduction buildings in that dwarven cities. With default tax rate, maximum garrison and a temple, you should be able to have up to 12 units of population without rebels. A decent production speed should be manageable.
Remember that the most special units of the dark elves are the night blades and the warlocks. Warlocks because of their doom bolt spell ability, night blades because of their invisibility. Also try using dwarven engineers for road building. Since all roads on myrror are enchanted, (free movement) and dwarven engineers build roads at double speed, that gives you a massive tactical advantage. To prevent the enemy from using this network against you by instantly reaching undefended or poorly defended cities, strategically place lone night blade units at choke points in your road network. They cannot be seen, thus cannot be evaded. After accidentally attacking them, the enemy cannot move any further and thus can be intercepted by your armies in the next move. And due to invisibility, you night blade should be able to survive just fine.