I'm confused by the Shapechanger ability, which seems to be all over the place for different creatures. Here's the description of the ability from the Succubus stat block:
Shapechanger. The fiend can use its action to polymorph into a Small
or Medium humanoid, or back into its true form. Without wings, the
fiend loses its flying speed. Other than its size and speed, its
statistics are the same in each form. Any equipment it is wearing or
carrying isn't transformed. It reverts to its true form if it dies
Normally I'd assume that means she can choose any form at the time she uses the ability- she could become the barkeeper in the tavern, or a small child, or one of the party members- whatever suits her schemes. But contrast that with the Doppelganger, which has slightly different text for the same ability:
Shapechanger. The doppelganger can use its action to polymorph into a Small or Medium humanoid it has seen, or back into its true
form…
The fact that it's different got me wondering why they didn't use the same ability description for both abilities- does that mean the Succubus could shapechange into a humanoid she hasn't seen?
Then there's the Slaad, whose ability description in the stat block is identical to the Succubus, but has the following line in their flavor text:
Shapechangers. Some slaadi can transform into the humanoid creatures from which they were originally spawned. These slaadi return
to the Material Plane to sow discord in the guise of their former
selves.
So that implies they have one form they can change into, even though it's not stated in the stat block- but there are creatures who can only shapechange into a unique form, such as the Jackalwere:
Shapechanger. The jackalwere can use its action to polymorph into a
specific Medium human or a jackal-humanoid hybrid, or back into its
true form (that of a Small jackal)….
OK, so since Jackalweres can only transform into a specific humanoid, that implies that everything else can choose ANY humanoid form, right? But then there are Lycanthropes, the classic shapechangers, whose stat block reads:
Shapechanger. The wereXXX can use its action to polymorph into a XXX-humanoid hybrid or into a XXX, or back into its true form, which
is humanoid…
I'd always assumed that Lycanthropes can only assume one specific form when they change, even though it's not stated in the text- for example, when Stuart the Wereboar Merchant takes his hybrid form, it's half Stuart/half boar, not any half-humanoid boar form. He couldn't become half boar/half Beauregard the Ranger to throw off suspicion, for example. That would make the ability more like that of the Jackalwere, although for some reason Wizards chose not to specify that.
This leaves me second-guessing my assumptions regarding Shapechange- does the ability a) allow the creature to take any other form unless specified otherwise, b) allow the creature to take a unique second form unless specified otherwise, or c) you're giving Wizards too much credit, and looking for consistency will drive you mad?
Since I'm writing an adventure for publication with a succubus villain, I can't fall back on DM fiat- if there's a consensus on this, or a clarification somewhere, my whole adventure will fall apart if I get it wrong.
Best Answer
Different creatures, different abilities, different rules
While all of these fall under shape-changing, each creature/ability/spell has its own set of rules.
This is a great example of specific over general with regard to the ruleset. Shape-changing is a general rule allowing a creature to change their form, while each instances of Shapechange/Polymorph has specific rules that apply.
When in doubt, do not apply general rules to a specific creature - just use the requirements and limitations stated for that creature.
Creature type vs Ability
Do note that there is a difference in the Creature tag Shapechanger and the ability Shapechanger. Creature tags are important to determine if certain spells/abilities will affect a specific creature (e.g. Polymorph doesn't work on shapechangers). The Shapechanger ability describes something that a creature can do and is dependent on their stat block with regard to how that ability works (as seen above, different creatures change shape differently.)
The Succubus
As you are looking for specific direction on the Succubus (MM, 285) for your campaign, then you need only refer to the specifics of their statblock (emphasis mine):
Given that there is no requirement for a specific Small or Medium humanoid, it can take the form of any Small or Medium humanoid. Tricksy little fiends (note: they are not of the shapechanger creature tag, but are a fiend as stated in their stat block)!