No, being skinny does not make breast augmentation dangerous at all and most women who get breast implants are quite fit with very little breast tissue.  That is the reason they decide to have breast augmentation.  Your concerns about being top heavy are well founded, especially if you see a surgeon who makes the breast implant size decision for you rather than you trying different size implants in a sports bra and making your own decision.
I would also recommend the shaped, natural feeling, gummy bear implants by Sientra which have the lowest complication rate and by far the best warranty of all breast implant manufacturers and here is an article on why they are so good.