The massive MIMO can operate in the sub-6 GHz and in the mm-wave and each of them has its suitable model. Dr. Emil suggested one for mm-wave operation. you can decide which frequency you will select and for which application you will plane to design, then select precisily the suitable masive MIMO.
Yes. Rayleigh fading is the best but you can have four scenarios of Rayleigh fading such as flat, frequency selective, slow and fast fading channels. Best of luck