PCI or PCI-Express, does it matter?

In reviewing tens of articles on best motherboards, I've not found a conclusive answer to the question:

Do you need all of the GPUs to be hooked up via PCI-Express? Does it matter?

For example, I found the Gigabyte GA-Z170XP-SLI Socket LGA 1151 (here), it has the following expansion slots:

Expansion Slots:
1 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.
1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot will operate at up to x8 mode.
1 x PCI Express x16 slot, running at x4 (PCIEX4)
* The PCIEX4 slot shares bandwidth with the PCIEX1_2 slot. When the PCIEX1_2 slot is populated, the PCIEX4 slot will operate at up to x1 mode.
2 x PCI Express x1 slots
(All of the PCI Express slots conform to PCI Express 3.0 standard.)
2 x PCI slots

So in total this will allow me to hook up 7 GPUs, but will this work?

Thanks