The "ally" sheet is simply an NPC sheet with a silly name. It's just a cut-down character sheet that fits two on a letter-sized sheet of paper. Sadly, it seems to have been accidentally misplaced or deliberately removed during one of PegInc's habitual, link-destructive website reorganisations. Since there's nothing really special about it though, any of the fan-made ally sheets should be functionally interchangeable with the official one. You're not missing much of anything by not having access to the PegInc original.
The ally sheet has almost everything that a normal character sheet has, except that there are fewer blanks since NPCs usually have fewer skills/Hindrances/Edges/Powers to track than PCs and don't change as much, if ever. The things it's missing aren't really notable – encumbrance stuff, a space for Charisma, gear location tracking blanks — trivial things like that. It doesn't have anything special on it apart from a bunch of general-purpose blanks in the middle that are suitable for notes, personality, and other miscellaneous things that are important for specific NPCs but don't deserve a dedicated blank on every NPC's sheet.
For an important NPC that's expected to evolve over the length of the campaign, a full character sheet would be more suitable. But for every NPC that's less detailed than that, from villains down to recurring torch-bearers, the ally sheet is plenty.