I'd say don't worry too much about the scale. While it would be nice for all models to be true to scale it's also highly impractical.
You'll also probably thank yourself later as you'll open yourself up to the possibility of using other bitz in the conversion.
As for body armour,
SoB wear power armor and I've always seen
SoB as wearing a ceramite plated corset with metal boning, studs, and edges. I don't really think leather or other materials work as
SoB PA is supposed to provide the same level of protection as suits worn by Space Marines.
No doubt you've seen this artwork:
While it looks similar to skin tight leather you'll notice that at the joints it shows to be more like body-fitting sections of armor
Note that the corset doesn't come down past the hips on the sides. The particular shape allows for leg mobility so if you wanted to model a belt onto a sister I think either a belt that follows the shape of the corset, goes over the corset or cuts the bottom of the corset off could all work well.
Maybe look a some pictures of women wearing corsets to get a general idea of body motion/movement while restricted by that particular piece of clothing?