What will a 170 cc over the muscle, low profile gummy bear implant do for me? Will my breasts look natural and not stiff?

Thanks for your question! Very small low profile implants won’t do that much for you.  Particularly the low profile part.  Moderate projection would likely do very nicely given the look you are interested in and based on your photos. Small implants could be fine if you are petite in size and stature, but I can’t say for certain without  examining you in person. The “gummy“ part is fine as long as you choose the softest one it will not be stiff, unless you develop a capsular contracture. 
Although you state that you don’t want implants placed under the muscle, you should know why going over the muscle is not recommended. This will predispose you to the development of capsular contractures.  While there are certain factors that we don’t understand regarding why capsular  contractures happen and in whom, placement of the implant above the muscle is something that we do know and should control for whenever we can. That’s not to say that you would get one for sure, and if you did it might be very mild, but we have no way of knowing. As long as you understand the risks involved, you can plan as needed. 

You need a formal evaluation by a licensed plastic surgeon and expert in breast augmentations to determine what is best for you and your goals.
Proper breast enlargement is not a "cookie-cutter" procedure where the same technique is applied to every patient. Every woman who comes to my practice has unique needs and different goals for her breast augmentation. As a board-certified plastic surgeon, I always listen carefully to each woman's concerns and thoroughly discuss the key points of the procedure. I also take into account their overall height and weight, the size and shape of the underlying chest and ribcage, the tightness and elasticity of the skin, and the amount of natural breast tissue. Prior to the procedure, we discuss and make decisions regarding location of the incision, whether to place the implant under or over the chest muscle, and what type of implant to use (I offer multiple types of silicone gel breast implants, as well as saline implants).
My experience performing thousands of aesthetic breast surgeries in a wide anatomical range of patients enables me to achieve consistently beautiful and natural results that are customized to each patient’s body type and goals. These results are the product of extensive communication, meticulous planning, and skillfully executed surgery.
I suggest consulting with a board-certified plastic surgeon and expert in Breast Augmentation surgeries. - Dr. Cohen

Thank you for the question and pictures.  Based on your photographs, I think that you are starting at a good place and should have a very nice outcome with breast augmentation surgery.  There seems to be is a moderate amount of breast asymmetry and the use of different sizes of breast implants may help to improve overall breast symmetry.Much of the final “look” and feel achieved with breast augmentation surgery depends on several factors:
1. The initial shape, size (volume of breast tissue), symmetry of the patient's breasts. In general, the better the preoperative breast appearance the more likely the breast augmentation “look” and feel will be optimal.  
2. The experience/skill level of the surgeon is important in determining the final outcome. For example, the accurate and gentle dissection of the breast implant pockets are critical in producing long-term well-placed breast implants. I personally think that these 2 factors are more important than any others, including type (saline or silicone) or model (low/moderate/high profile) of implant.
3. The type of implant used may determine the final outcome, especially if the patient does not have significant covering breast or adipose tissue. For example, some surgeons feel that silicone implants have a more natural look and feel than saline implants because silicone gel has a texture that is similar to breast tissue.  In other words, if a patient has very low body fat and/or very little breast tissue, the silicone gel implants may provide a more "natural" result.
4. The size and model of breast implant used may make a significant And then you open a website run at 866difference in the final outcome. Therefore, it is very important to communicate your size goals with your surgeon. In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C or D cup” etc. may mean different things to different people and therefore prove unhelpful. Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup size may also be inaccurate.
  
I use intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the breast implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible. I hope this helps. Best wishes.