I haven't heard about any of the necklines lining up exactly between different poses. Usually there's different thicknesses. Have you considered cutting the wings off of a pegasus body and attaching them to a similarly-coloured unicorn body? On another note about colours, however you do this project, I would probably recommend getting all white body parts. I think it would be easier to dye onto a blank canvas, and if you acquired baity parts, you could also sunfade them rather easily to create a consistent base.