Can I achieve a nice slope and voluptuous results when starting with small and flat breasts? (photo)

Hi there,
Post some photos of the results you are looking for and we can help better.
In general, yes you can achieve the results you want. You will not look overly round or fake if you go under the muscles and choose the implant based off your specific breast and chest wall measurements. This will be discussed with you at your consult.
IF you want very larger breasts you may have to get there in two surgeries. The first surgery with a more modest implant size, which will stretch the skin and shape the breasts, then a few months to years later do a remove and replace to a larger size.
It really depends on how large you are talking however. You don't want to put an overly large implant in for your first time as the amount of stretch your body has to undergo can be unpredictable and lead to asymmetry or bottoming out.
So it is certainly possible for you however may take two surgeries to get there. Be sure to take photos to your consult of what you are looking for
All the best

I would go see a board certified plastic surgeon (American Board of Plastic Surgery) with preoperative 3D simulation capabilities (Vectra, Crisalix, etc) to be able to show you the outcome.