[Serpion5]

To be completely honest, if you're going for a Chaos Sisters look I'd probably leave the shoulder guards pointed. I'd be trying to keep those arms intact as well, and only cut out the crossbow itself if that's possible. I'm not sure how the kit goes together but I think the space marine hands and arms will be too huge for those guys. By the looks of the sprue on the site the hands and arms are attached to the crossbows so that could be a pain.

Which backpacks specifically do you plan to use? The normal tactical squad ones or something more elaborate?

 

[Serpion5]

The problem @shaantitus is that most gw stores now have a gw model only policy for gaming in them. Some store managers might be a bit lax, but in general they consider games in store to be a part of their image so they require that players only use their product, and preferably current models too but I don't think they enforce that bit too harshly.

Dark Eldar torsos are going to be a pain. From memory there are only four female tosos out of ten on the sprue and it could in fact be less than that, I'm not sure. Additionally, the models themselves simply don't have the right kind of base to be used as sisters. I think your original plan of using the Darkshards is a safer bet.

Keep in mind they'll be holding bolters in front of them so you shouldn;t need to worry too much about making the breast on the armour too pronounced. Real word armour worn by women doesn't follow that shape, and the heads you've chosen will make it apparent that they're women anyway.