25 y/o who wants kids in future. I would prefer a fat transfer but am concerned I am not a candidate and not sure about implants

Unfortunately, you have not provided photographs.  The technique I recommend is The Bellesoma Method.  This will reshape your breast tissue creating upper pole fullness without implants, elevate them higher on the chest wall and more medial to increase your cleavage.  Vertical scars are avoided, nipple sensation and the ability to breast feed are maintained.  At the same time or later, fat transfers can be performed if additional volume is desired.Best Wishes.Gary Horndeski, M.D.

I would suggest meeting with a board certified plastic surgeon, being examined, and discuss your options. Even with a fat transfer, you may only achieve up to one cup in size. If you want more, implants would be best.

Recommend that you schedule an in-office consultation with a board certified plastic surgeon who performs hundreds of breast procedures annually.  This is the only way you will be able to obtain an objective opinion regarding the best plan for you to achieve your ultimate goals.
In general, 200 cc per breast is typically the upper limit of fat injection recommended by most plastic surgeons.  It is not that more cannot be injected; but on average, this has been determined to be the most that can be expected to live during one transfer.  About 50% of the fat obtains permanence.  The fat must be injected through multiple planes in small amounts to the tissue through one tiny incision under each breast.