RE: Soulfire
I read the quote like this:

“Heavenfire?”
“Well…” Bob said, “yes. And no. Hellfire is something you use to destroy things. Soulfire is used the opposite way—to create stuff. Look, basically what you do is, you take a portion of your soul and you use it as a matrix for your magic.”

As in: Soulfire serves a similar, opposed function as Hellfire (which is the 'yes'-part), but unlike Hellfire it is power within you, used and controlled by you for your purposes.

And compels would be similar to Harry's exhaustion after unintentionally blasting those RCVs and Bob remarking how his soul was shredded because of it.

1. How did you start gaming?
A few people I was playing badminton with invited me to try it.

2. What system did you start with?
Dungeons and Dragons Third Edition

3. What systems have you played?
Dungeons and Dragons Third Edition, Dungeons and Dragons Fourth Edition, FATE, Savage Worlds (orig., revised, explorer's, deluxe), World of Darkness (old and new), Exalted, SIFRPG, Shadowrun.

4. What system/s are your favorite?
In general, FATE has a lot of appeal.
Shadowrun is the sort of setting where the system fits the setting amazingly.
D&D3e has a special place for me, of course.

5. What caused you to try DFRPG?
I heard about it and bought them on a whim Then, when I was moving (involving a lot of trips with the train), I bought some books to read, including Storm Front. Two months later, I'd read them all and looked over the roleplaying system from a new perspective. Then I tried getting a group started up and that brings us to the present.

6. What is your preference in gaming group size?
I've had groups between 2 and 8. Each type of group lends itself to particular types of games, so I fit the game style to the group size.

7. How open are you to gaming with people you don't know or hardly know?
Fairly open, though if a total stranger stopped me on the street and asked I'd be creeped out that he knew I was a roleplayer.

8. What is you best quality as a GM?
Willing to fit plotlines to whatever the players want.

9. What is your worst quality as a GM?
I'm **** at preparing for sessions.

10. Importance of game balance 1 - 10 (1 being lowest 10 besing highest)
5.
Game balance needs only be observed if the players care. Infinite loops or malicious reading of the rules are banned, but if you've found a superior strategy, then, by all means, use it.

11. Importance of setting/canon of universe 1 - 10 (1 being lowest 10 besing highest)
#.
Depends on what the group agrees on.

12. Custom setting or established setting?
Yes.

13. During character creation are freedom of powers more important or the chracter concept and the powers and skills fitting that concept?
I expect the characters powers/skills to fit their concept. Whether powers inspire concept or concept inspire powers, I really don't care.

17. Roll play or roleplay?
Stormwinds Fallacy.

A storm capsizing their ship might have skills like "Treacherous Undercurrents (Grapple)," "Black Depths (Attack, Weapon: X)" and "Violent Waves (Maneuver)" which would each have a rating and function as an aspect. The players can take it out by making successful rolls against its defense, dealing stress. Consequences would be stuff that makes it easier to deal with whatever else I'm throwing at them at the time.

Meanwhile, jumping into the ocean to battle and capture a Fomor they've been chasing might simply be an environmental aspect to tag, invoke or compel.

It depends completely on the context and whether I've had time to prepare

Well, since my DFRPG group is down to two players, I've been a bit hesitant to do so. Not much opportunity for PC-PC arguments if the other isn't present, now is there? 

That being said, I don't worry about it. As long as the players are split, I get to do that whole "switch scenes at a tense moment" deal I like to an unhealthy degree. Besides, as the old saying goes "Let's split up! We can take more damage that way." 

It has already been said that some powers are quite generic. Strength, Speed, Toughness for example are so basic that their expression in the game can vary a great deal. Others are a lot more loaded with innate "flavor". Blood Drinker or Addictive Saliva for example.

Well, if I was to mimick the Blood Bond from World of Darkness vampires, I'd use Addictive Saliva to mimic that. Same for Stargate SG-1-esque mind-control devices (melee range, little physical harm, sometimes addictive).
Or how about being a demon, whose touch attempts to rip the soul straight out of your body (thus, mental damage)?

The Swords of the cross are a good example here, as is the Sacred Guardian power mentioned by the op.

Actually, if people have a proper concept and are willing to pay an increased price, I'd gladly let them buy, for example, All Creatures are Equal Before God.

So if you're the Scion of the Grim Reaper and your goal is death, death, death (Also known as a "Killf**k Souls**tter") I might let you take something like:

• Claws + Venomous.
  You touch someone and their souls start rotting, represented by pain and agony until they're either Taken Out or remove the aspect.
• Sponsored Spellcasting: Death.
  I think there's a write-up on these boards. Otherwise, I'd wing it.
• All Creatures Are Equal Before God.
  I might rename it "One Day, I'll Even Reap God." Otherwise, it stays the same.
• Living Dead.
  Because, barring strange aeons, death won't die.
• The Sight.
  As if a Veil would save you from Death.
• Ghostspeaker.
  Arguably unrelated, though if the player asked for it, I'd gladly give it.
• Greater Glamours.
  Invisible, unnoticed and make a scythe appear out of nowhere? Check.
• Supernatural Sense.
  Seeing when people are to die, seeing souls, sensing life/death.

Now, he's got powers from the Minor Practitioner, Emissary of Power, NPC-only and Knight of the Cross Templates and still works just fine. Sure, he could be called a form of Changeling, Scion or Emissary, but who cares what metagame term is used to describe his "I deal with death"-sthick?

If I was to "upgrade" Cassandra's Tears, it'd be something like this:
[-1] - You are no longer at -2 to convince people of your prophecies.

Then add on "A Few Seconds Ahead" (Our World, Abby from the Ordo Lebes) and (possibly) Guide My Hand.

Negotiable add-ons: Psycometry, The Sight and Soulgaze to finish the "I know EVERYTHING" character.

Eh, I was away from the books, so I just slapped it together. Add Inhuman Speed and Strength, if you want. And, by the way, this "horde-unit" isn't supposed to work alone. I wouldn't "field" them unless there were at least 4 groups and still, they would only be there to make things interesting until the Necromancer(s) show up.
They're basically swarm units by virtue of more stress track, a bit of armour and being able to split attacks.