Enemy FATE Points
DFRPG comes with one specific scaling mechanic - NPCs and Fate Points on page 351 of Your Story suggest that you can be lazy and pool all NPC Fate Points. At the start of each session, you take one point per player, spend from that pool for any NPC and add to it when PCs compel and invoke to disadvantage NPCs.
This Fate pool cannot be added to if it would end up with more points than the total amount currently held by the players. If you would have to do this, just ignore the point and carry on.
The other useful scaling mechanic for encounters is grouping the minor NPCs. This comes from Spirit of the Century and FATE Core. SotC has Minions and FATE Core has Mobs of Nameless NPCs and advice on right-sizing.
In both of these cases, minor NPCs are used as the fibre of a good conflict meal. They help bulk out a group and provide a little cover to any supporting and main NPCs.
In DFRPG, the Scaling the Opposition rules on page 331 suggest the total amount of refresh opponents should have for a given group. You can re-calculate the guideline amount at the beginning of a session and use this to buy relevant NPCs for a conflict like D&D 4E's experience budget.
For example, if you have a group of 3 whose total refresh spent on powers is -12 and -2 on mortal stunts - you would design encounters based around -10 for this session. A minor challenge would have opponents with a total of -5 of refresh in powers, equal -10, major -15 and overwhelming -20.
If they were going up against a cartload of chimp-sized Shen (-3 refresh, page 35 of Our World), this group should face from 1 to 6 Shen in the cartload. Next time, a full wizard has joined them with -7 refresh. This means that a good number of Shen is somewhere between 3 and 12.
To reduce the number of rolls you are making, these Shen should be collected into groups the same size as the number of players. With the three players, this would be 1-2 groups of 3. With four players, this would be 1-3 groups of 4.
@Magician makes a good point when he says that you shouldn't make Personal Plots a heavy part of the game because other players can't be as involved with them. I think he goes too far by saying you shouldn't use player-specific plot at all. FATE highly encourages players driving the plot and them building stories based on their entire character.
Taking the metaphor of a TV show further, look at the ensemble shows and how they join and separate characters while their plots progress.
You can bring personal plot to the fore by restricting major uses of that plot to the originating character but allowing minor usage around the characters that have been exposed to that plot.
As an example, consider a game including an actor, a surgeon and a lawyer. The actor has added “Debt to the Mob” to his backstory and is joined to both the surgeon and the lawyer through “Old College Friendships”. The surgeon and the lawyer are “Fighting the Man” in a courtroom battle.
While the actor is playing, it’s valid to bring the mob to bear. The Italian Restaurant the surgeon takes his old friend to turns out to be a mafia hang-out for instance. Once the surgeon has been exposed to this, it makes narrative sense for him to discover that the prosecutor in his case is taking bribes from the crime family involved.
While the actor is not playing, the mob should be on the backburner. It’s there but the other players have more pressing concerns from their own stories to contend with.
Before you engage with this though, there is something the whole should agree on – nobody tries to resolve anybody else’s story for them. Important NPCs for player’s who aren’t present must be left alone.
SotC also has advice on the pick up game in chapter 9. This is mostly for the pulp genre, but the advice should be exportable to your situation.
: Page 221 of the Kickstarter draft. Reference should still be valid in the final version.
: Page 228 of the Kickstarter draft.