I compared a Star Wars Legion figure to one of my borther in law's Primaris Marines a while ago, and they sat beside each other quite happily.
I can't find that figure at the moment (it's deep in the cupboard of shame), but I've just compared a Necromunda Delaque with a Star Wars *Imperial Assault* (28mm, not Legion) figure, and the Delaque is a full head and shoulders taller. However, not all Necromunda figures are that tall, and I seem to remember the Van Saar are actually quite short.
Others will hopefully be along with a bit more objective advice, but I suspect you'll get away with it.