How to declare a pointer for arduino neopixels library so I can set it up in the setup function with dynamic variables?
You can assign the pointer to NULL, and in the setup function give it a value. Note you have to add () for the arguments, even if there are none like in this case. Also you have to include the library (but probably you did but not added in your question).

#include "Adafruit_NeoPixel.h"

Adafruit_NeoPixel* pixels = NULL;

void setup() 
{
  pixels = new Adafruit_NeoPixel();
}