As to formulas, here are some best guesses so far. While individual formulas may be controversial, I haven't yet figured out a system that works better for both Pit Fiend and Spined Devil without relying on unprecedented, arbitrary bonuses (although, I'll admit, I'm only vaguely familiar with SWSE).
Note: these equations all assume an Ability Modifier is calculated the same as in 3.5. I'm 99% certain that is the case, since alternate methods cause weird anomalies when applied to one monster or the other. For example, if the Pit Fiend had a +14 Str, then it would be a weird that his melee damage bonus is only 11. It makes far more sense that he has +11 Str.
Anyhow...
Initiative: Cha + 1/2 Level
Pit Fiend: 9 + 13 = 22
Spined Devil: 2 + 3 = 5
Perception: Wis + 1/2 Level + (Trained?)
Pit Fiend: 5 + 13 + (5?) = 23
Spined Devil: 2 + 3 + (0?) = 5
Skills: Ability + 1/2 Level + Trained
Pit Fiend:
.....Bluff: 9 + 13 + 5 = 27
.....Intimidate: 9 + 13 + 5 = 27
.....Religion: 6 + 13 + 5 = 24
Spined Devil:
.....Spot: 2 + 3 + 5 = 10
Melee attack: Str + Dex + 1/2 Level
Pit Fiend: 11 + 7 + 13 = 31
Spined Devil: 4 + 2 + 3 = 9
Melee damage: Str
Pit Fiend: 11 = 11
Spined Devil: 4 = 4
Ranged damage: Dex
Spined Devil 2 = 2
Poison attack: Con + Con + 1/2 Level
Pit Fiend: 8 + 8 + 13 = 29
Fear attack: Con + Cha + 1/2 Level
Pit Fiend: 8 + 9 + 13 = 30
(I tried to figure out the Spined Devil's Spine Rain ranged attack. The problem is, in the old card it's listed as "+9 Dex vs. Ref"; whereas, in the new card it's listed at "+8 vs. DEF (Ref)". The +9 could have been Str + Dex + 1/2 Level, just like a melee attack against AC. But where is that +8 coming from? It could be Dex + Level, or it could be Dex Check + 1/2 Level, or it could even be Str + Str. None of those make much sense if we assume the other attack or save formulas are correct. They seem to be following an Ability + Ability + 1/2 Level method now.)
Fort: 11 + Con + Con + 1/2 Level + Saving Throw
Pit Fiend: 11 + 8 + 8 + 13 + 2 = 42
Spined Devil: 11 + 2 + 2 + 3 + 0 = 18
Ref: 11 + Int + Int + 1/2 Level + Saving Throw
Pit Fiend: 11 + 6 + 6 + 13 + 2 = 38
Spined Devil: 11 + 2 + 2 + 3 + 0 = 18
Will: 11 + Wis + Cha + 1/2 Level + Saving Throw
Pit Fiend: 11 + 5 + 9 + 13 + 2 = 40
Spined Devil = 11 + 2 + 2 + 3 + 0 = 18
AC: 11 + Dex + Dex + 1/2 Level + (Net Armor and Penalties?)
Pit Fiend: 11 + 7 + 7 + 13 + (6?) = 44
Spined Devil: 11 + 2 + 2 + 3 + (2?) = 20
(The 11 balances the aggressor's d20 roll, such that the aggressor wins a tie.)
-ALTERNATIVELY-
Ref: 11 + Dex + Int + 1/2 Level + Saving Throw + (Size Modifier?)
Pit Fiend: 11 + 7 + 6 + 13 + 2 + (-1?) = 38
Spined Devil: 11 + 2 + 2 + 6 + 0 + (0?) = 18
AC: Ref + (Armor?)
Pit Fiend: 38 + (4?) = 44
Spined Devil: 18 + (2?) = 20
(Of course, we don't know what armor bonus, if any, or dex penalties, if any, are given by the Pit Fiend's breastplate or the Spined Devil's spines or whatever. So AC may be impossible to verify before the release date. Reflex should become clearer after the next monster preview.)
HP: [(Con + d10 average) * Level] rounded down to the nearest 5.
Pit Fiend: (8 + 5.5) * 26 = 351 -> 350
Spined Devil: (2 + 5.5) * 6 = 45 -> 45
(This formula is almost certainly wrong. Although the Spined Devil has 45 hp in the updated minis card, it most likely has 47 hp before rounding, given the info from the older card. In any case, the correct formula is difficult to figure out without knowing how hp varies with monster type, role, or rank. We need more monster previews with stats.)