Can I do 1.5 mls of fat grafting to my labia majora to give me the natural and firm look that I'm looking for?

It is possible to get fat transferred to your labia majora, however this should be performed after the effect of the fillers has disappeared. Your surgeon can then provide you with specifics on your fat transfer procedure, and achieve more predictable results.

It may be a good idea to take photos of your labia majora at this time, while the effect of the fillers are still visible, so you can show them to your surgeon. They can then better understand what you’d like to achieve.

It was our experience that fat grafts in labia do not stay for long and do not take well. One has to significantly over-graft (inject more graft than needed) to expect visible result in 5-6 m. Fillers may be a better choice for small volume augmentation?

Unfortunately there is no volumetric equivalency formula for fat grafting compared to fillers.
There are numerous variables to take into consideration such as the quality of fat, how the fat was harvested, had the fat was processed, how the fat is injected and the abundance and quality of host tissue. Each of these variables has a major impact on results.
If you concerned about being overtreated then you should consider having conservative grafts with repeat sessions in six months. most likely 1.5 mL of fat would be insufficient.
Talk to surgeons who have experience with the procedure and ask what volumes they use then ask to see long-term before and after pictures.
Best,
Mats Hagstrom MD