Second what Primetime said ..... Yes, you can keep 5 or 6 babies in 15 gl, but you will have problems within weeks or months. The only way this will work is if you treat the fish as disposable & replace them every few months when they die..... <NOT recommended!>
I would not keep commons in a 15 gal - for optimal results, you need 1 sq foot of surface area per common for swimming room. You can keep a couple fancies in there, but if you are keeping it outdoors, suggests moors or metallic fantails. They are the only fancies hardy enough for outdoors.