I've seen bridesmaid's have the same color of dress, but a different style on each and I think that is fine, it doesn't look bad. The more traditional way of doing it is of course to have everyone wearing the same exact dress. I think though if you want each girl to wear what they are most comfortable in, as long as they're all the same exact color it would be okay. I don't see any harm in it. Some of the girls may not want a strapless dress or may want to be more covered up or wear something that flatters their body type and that will make them look better, so I think it's a good idea if you have a lot of different body types for bridesmaids.
My feeling on this is that it's your wedding and should reflect your personal style. I have seen beautiful ceremonies where the girls wore different colors, styles, etc. I have also seen different dresses that were tied together by some small element like a belt or wrap in a matching color. Don't be afraid to be creative and have the ceremony that matches your style and that of your beidesmaids.
I think your bridesmaid dresses can be anything you want. It is your wedding. Several years ago when I got married I did the rainbow effect with my bridemaid dresses. I had heard about that somewhere and thought it was a cool idea. I love color, so it worked well for me. Do whatever you wish! Iti s your special day, not anyone elses!
Many times bridesmaids will have very different body types and the only decent thing to do is allow them to at least pick out a dress that suits their shape. These girls are not all going to look in the same style (heavy on bottom, tiny waist, large breasts, etc.). You can just pick a color and let them have at it, or you can guide them more than that.