I think it all works quite well, because it is all electronic goods are made to be used (just kidding: V). But one thing that you should consider before using the PCIe extensions, because these products have a maximum limit and also needs a very large when you use a GPU that has a high power level. For mining, would be much better you do the experiment well in order to better understand.
PCI riser will be takes to the 6 cable line for the 6 cards so you will be able to mine the ethereum and other top altcoins which can be minable.
Multiple GPU rigs will be give better profit with the domestic or commercial electricity.
If you mine any altcoins in your house itself you will be able to get more profit than the commercial current per KW.