How does one mix gyaru fashion with an outside style?

Honestly, anything can be gyaru as long as your hair and makeup read as gyaru.  Substyles are helpful, but don't let them limit you; They can be a helpful starting point, but don't think about it like a necessary box or label.  Wear something cute, something that you like the look of, something that you feel good in, and just top it off with some gyaru makeup and hair.  I tend to wear outfits that I think are cute, and just do my hair and makeup in a more gyaru way and it tends to look nice, I think :)  Hope this helps!!