You're not going to get fast demons outside of Kaalia, and she has a bit of a... reputation as being too strong not to kill. If you play with groups outside of your fiance and her brother then I'd avoid her. If you only play with those two then the onus is on you to make sure your deck remains fair since I'm assuming theirs are not particularly competitive.
I would say your best bet is to run someone else alongside a fair assortment of kill spells to stay in the game until you can start laying some demons down later on.
Of course, as a monoblack deck you'll want to run things like Nevinyrral's Disk, Oblivion Stone, Unstable Obelisk, and possibly Scour from Existence just to deal with critical artifacts and enchantments (better options than these also exist, but get into higher price ranges).
One of the best strategies for midrange, I've found, is to run sweepers for creatures smaller than what you're running. Demon of Dark Schemes, Yahenni's Expertise, Night Incarnate, and Toxic Deluge (that last one is fairly pricey, but very worthwhile) can kill your opponent's things while leaving your own mostly untouched. I've found the sweetspot is usually -3/-3 or 3 damage, as that kills most utility commanders and creatures while most demons have more than 3 toughness.
Liliana's Contract is probably the best 'demon tribal' card out there, largely for lack of competition. Remember that a demon token counts as a demon named "Demon". Blood Speaker and Bloodthirsty Ogre are the next best.
There are lots of demons out there that like eating (or otherwise sacrificing) creatures smaller than them, such as Demonlord of Ashmouth, Demon of Catastrophes, or Demon of Death's Gate, although they don't play well in a deck that's heavy with other big demons which you don't generally want to sacrifice. If you want to run some zombie token generators then those demons will play nicely with them, and the zombies are also a good way to hold off the early-game legions until you can start playing some big meanies.
I would say your best bet is to run someone else alongside a fair assortment of kill spells to stay in the game until you can start laying some demons down later on.
I've had positive experiences with both Ob Nixilis of the Black Oath and Kagemaro, First to Suffer, both as control-heavy black decks.
Ob Nixilis appreciates good removal and good blockers (Wall of Souls, Vampire Nighthawk, Will-o'-the-Wisp, Mischievous Poltergeist), and in return gives you good life management to stay in the game and comes with his own demon buddy/buddies for additional blocking.
Kagemaro appreciates being run with relatively few creatures, more card advantage spells (Night's Whispers, Read the Bones, Promise of Power), and ways to easily get her back from the graveyard (Animate Dead, Undying Evil, Disentomb, Reaping the Graves, Death Denied). She can be a bit unfun to play against for certain decks though.
Of course, as a monoblack deck you'll want to run things like Nevinyrral's Disk, Oblivion Stone, Unstable Obelisk, and possibly Scour from Existence just to deal with critical artifacts and enchantments (better options than these also exist, but get into higher price ranges).
One of the best strategies for midrange, I've found, is to run sweepers for creatures smaller than what you're running. Demon of Dark Schemes, Yahenni's Expertise, Night Incarnate, and Toxic Deluge (that last one is fairly pricey, but very worthwhile) can kill your opponent's things while leaving your own mostly untouched. I've found the sweetspot is usually -3/-3 or 3 damage, as that kills most utility commanders and creatures while most demons have more than 3 toughness.
Liliana's Contract is probably the best 'demon tribal' card out there, largely for lack of competition. Remember that a demon token counts as a demon named "Demon". Blood Speaker and Bloodthirsty Ogre are the next best.
Some good demons that aren't too pricey...
Abhorrent Overlord
Archfiend of Depravity
Bloodgift Demon
Demon of Wailing Agonies
Desecration Demon (not actually that good, but can be fun)
Doom Whisperer (~$5)
Harvester of Souls
Kagemaro, First to Suffer (if not your commander)
Kothophed, Soul Hoarder
Ob Nixilis, the Fallen (~$10)
Overseer of the Damned (~$4)
Pestilence Demon
Reaper from the Abyss
Seizan, Perverter of Truth?? (iffy, but fun; ~$5)
Sower of Discord
Spawn of Mayhem
Tombstalker
Vilis, Broker of Blood
Westvale Abbey (~$5)
Rune-Scarred Demon (~$5)
There are lots of demons out there that like eating (or otherwise sacrificing) creatures smaller than them, such as Demonlord of Ashmouth, Demon of Catastrophes, or Demon of Death's Gate, although they don't play well in a deck that's heavy with other big demons which you don't generally want to sacrifice. If you want to run some zombie token generators then those demons will play nicely with them, and the zombies are also a good way to hold off the early-game legions until you can start playing some big meanies.
- Rabid Wombat