Jonatan,
yes it's okay to have a mobster on your ambition front and a sickness on your decay front or whatever, even if both are inside the holding.
The rule for creating fronts is to link threats thematically (by scarcity), not necessarily in the setting (like 'inside the holding'). This is so you can have different threats in the same setting ('in the holding') hitting different themes. If the two threats are thematically-related (the Cloverfield monster and its flea monsters probably are) then they go on the same front. If they aren't thematically related (the ambitious mob boss and his hungry gang member) then they go on different fronts. The hungry gang member is on a front with a bunch of other threats that express hunger (the crew that works the primitive greenhouse, the cannibals, that rival holding out on the flats with nothing to their name, whatever).
When you make your mobster threat and create a front around him, some thematically-linked threats will come to mind that are also linked to this mobster (his lieutenant who wants to take over, or that scrappy street kid who's dying to get noticed by the boss and become a made man). When that happens -- great! Add them to the front. If nothing comes to mind and you need to fill up that front with other threats, invent some threats that also express ambition, whether they tie directly to the mob boss or not -- the DA who is running for governor and is making a show by cracking down HARD on the ghetto where the PCs live, maybe? (I know that's a little weird for AW; I'm assuming a Goodfellas-type hack of AW with mobsters and stuff). In the end, you have a front that expresses ambition seven ways to Sunday.
Sometimes you'll have a threat and won't know exactly what front it goes on (that is, you won't know what theme it expresses right off the bat). That's fine too. Throw it on the Home Front until you figure it out.
At the end of the day, you have all your threats organized by theme (as opposed to being organized by how they relate to each other in the fiction, or organized by where they live, or organized according to a million other schemas). This should hopefully make it easier to keep the themes your game is expressing firm in your mind, which in turn will hopefully make it easier to look for ways of expressing those themes. So every time that hungry gang member comes on stage, look for some way of letting his hunger (or someone else's, whatever) shine through by word or deed, for example. No one is going to go through your fronts and complain that you don't have the mob boss and his hungry gang member on the same worksheet. It's really there as a resource to help you, not to lock you into some rigid format, right?
Make sense?
-- Christopher
PS: I guess I should point out that in addition to theme (scarcity), the threats on a given front are also linked by the fact that they impinge on the PCs' lives. The ambitious DA in my silly example above needs to impinge on the PCs' lives (like, she needs to make waves where they live or something, not Back Home On The East Coast or whatever). I'm sure you knew that though, just clarifying.
EDIT -- re-reading what you wrote, I'm positive you already get most of what I wrote. Sorry for mansplaining it to you. I'll just leave it here in case someone else finds it helpful.