Hi URSTech,
Congratulations on your new mattress first of all … you certainly made a good quality/value choice 
If you let them know you were a member here when you made your order they may still be able to add the pillow to your order.
Post #2 here has more about the many variables that can act together to affect the sleeping temperature of a mattress. While the top layer of foam can certainly make a difference … your mattress cover and quilting materials, your mattress protector, your sheets and bedding and your pyjamas will all have an effect and layers or materials that are closer to your skin will have a bigger effect than layers or materials that are further away.
The gels that are added to foams are either thermally conductive or phase changing materials and can have a temporary effect on conducting or storing heat away from the body but the effect tends to be temporary and when temperatures equalize then the insulating properties of the foam would once again become more apparent. There is more about gel materials in post #2 here and the posts it links to.
Airflow and moisture wicking are the most effective method of regulating temperature and in general terms latex (Talalay more than Dunlop) is the most breathable of all the foam materials. Natural fibers such as horsehair, wool, and cotton or viscose materials such as rayon or bamboo that can wick moisture away from the body and store it inside the fiber are more effective at regulating temperature than any foam. Synthetic fibers such as polyester don’t absorb moisture well and tend to maintain higher humidity levels closer to the body and sleep warmer.
The first step I would suggest before adding any layers that you may not need at all would be to do a little bit of experimentation with the layers in the 12 in 1 and then decide if you need to add anything else.
Phoenix