PCIE x1 card in PCIE x16 slot?

Yes, any PCIe 1x, 4x or 8x card will work just fine in a 16x slot View full

For example, an x1 slot can have only one transfer lane used by the x1 card inserted into the slot; x2, x4, x8, and x16 slots are also available

1x slot casino PCI Express: Unidirectional Bandwidth in x1 and x16 Configurations ; Generation, Year of Release, Data Transfer Rate ; PCIe , 2003, GTs

191 slot In ASUS 「PCIe x16 Slots - 1」