no prob, im running 4 VEGA64 on A (incl the 16pin) and their risers on one PSU powering all mobo connections and 2 VEGA64 and their risers on C powered by a separate PSU
its important to use mobo screen output
dont use sata for risers, dont connect GPU direct to mobo 16pin
my path to wisdom is here:
https://bitcointalk.org/index.php?topic=2549085.new#new