Stocking for a 15 gallon aquarium

Question
I currently own a 15 gallon aquarium. I have already owned this for 6 months, but decided to add in more fish. I know my tank is not completely  stocked, but I am not certain how many more fish i can add to my tank. I am not completely certain about the 1 inch of fish per gallon rule, since generally an aquarium with a larger surface area will support more fish. So I decided to do some research to find a better stocking rule. This is the one I found: To find the correct stocking level, first calculate the the surface area by multiplying the tank length by the tank width. (My tank is  24 inches by 12.2 inches wide.) Then for every inch of adult body length will require 12 sq inches for tropical freshwater species. So is the surface level of my tank 292 inches, and is this rule correct.
Also can you keep a female Betta with a Dwarf Gourami? I don't think so, but wanted to make sure.
These are the current inhabitants of my tank....

Female Betta (1) 2 inches
Red Wagtail Platys(4) 8 inches total

I'm hoping on adding in some Julie Corydoras (2 inches per fish)

THANK YOU!!! : )

Answer
Hi Kesley!

Yes! The rule you saw online is correct but it depends on the size of the adult fish. And no, please refrain from keeping gouramis with a betta because they will fight each other and the betta might die.


If you have any more questions please follow up !

-Sam