"Humanoid" and "monstrous humanoid" are well-defined terms, meaning different things. That said, it would be cumbersome if written material could not use the word "giant" to merely mean "big" if it were clear from context that that was what was going on.
When the D&D rules say "humanoid," to refer to a creature, they clearly mean the creature type. If they mean otherwise, which is rarely, they generally take pains to not use the word to describe a creature, but a shape, as with the phrase "humanoid in form." So no, hold person and such do not work on monstrous humanoids.
With the succubus, however, the ability says they can take "humanoid forms," not "the forms of humanoids," so it could reasonably be taken as describing a shape rather than referring to a creature type. I haven't seen a clarification on the issue.