External display questions for Macbook Pro mid 2015 (DG)

I've been running two* external displays on my Macbook Pro mid-2015 with dedicated graphics** without a problem.

But now I have three questions:
1. I wonder If I also could be using the internal display at the same time? Or is there a risk of a heating problem?
2. Should I be using the HDMI port for the second display? Better for the machine/heating?
- Now I'm using thunderbolt port for the larger display and mini display port on OWC Thunderbolt 2 Dock.

3. What is the best setup for external displays on this machine?

I'm also using a stand with a fan under the MBPro.

* DELL 28,5" 2560 x 1080 and DELL 24" 1920 x 1200
** Radeon R9 M370X