NVIDIA cards will trottle and downlock if you don't have enough power. (If the power exceeds the TDP)

6 non powered 750 ti's on a single motherboard is not a problem as long as the motherboard is equipped with extra power. Like the h61 and h81 asrock BTC boards. Default tdp of the oldest non powered 750ti cards is 38w (in the bios) This can easily be checked with the NVIDIA inspector tool.