Having been a bridesmaid I can tell you that wearing a dress that does not befit your body shape and / or personality can be a BIG thing for some people (sorry to count myself among those).
My recommendation would be to meet half way, keep the color scheme the same and maybe have 3 different dress designs that your bridesmaids can choose from.
After all, you may want them all to wear that pretty dress with the spaguetti straps, but I'm sure your cousin who wears a 38 D would really appreciate it if she could wear a bra that offered some support, and like this, I could mention a hundred little things.
As you can see in this pictures, if you keep the color and fabric, you can achieve that "same dress" look, without having them wear the exact same dress... and of course considering that you can dress them up using different accesories to reinforce the look.
Here I'm posting different examples of bridal parties, enjoy. J...