I am running two rigs with this board using 6 gpus sucessfully and soon to test with 7.

I can tell you that you won't be able to use both of the M.2 slots as you hoped because atleast 1 of the M.2 slots shared PCIe lanes with the PCIex1 #2 and #4 slots on the motherboard.
I know this because to get 6 gpu's to run correctly you have to distinctly change that setting to disable the M.2 slot in bios and use the PCIex1 #2,4 instead.
I am not positive but you may be able to use the other M.2 slot. I will double check, if so I may just order an adapter and try it out.

Edit: I am wrong, I looked up the PDF of the manual. It turns out it was the front USB 3.1 connector I had to disable in BIOS that shares bandwidth with the PCIex1 #2,4. The M.2 slots only share bandwidth with SATA 5 and 6. The M.2 adapters may very well work.
       Link to manual (Section 1.6.7):  http://dlcdnet.asus.com/pub/ASUS/mb/LGA1151/STRIX_Z270E_GAMING/E12349_Z270E_GAMING_BIOS_EM_WEB.pdf?_ga=2.219611673.1480583767.1496203431-1198207353.1492379291