Succubus Alternate Form

[Kae'Yoss]

It says that a succubus can only change into humanoid forms, is this strictly "humanoid" or does it count other things like "monstrous humanoid"?

Humanoids only. Monstrous humanoids rarely are very beautiful (as the name implies), and therefore no succubus would want to take such a form. They could, IMO, take the form of something that looks humanoid but has another type, like planetouched (which have humanoids in their ancestry, usually humans).