Does any PCIe slot support NVMe SSD's automatically or do I need a specific M.2 slot?

All PCIe NVMe cards are PCI v3 (or later), although the PCI NVMe standard does allow for x1 connections the majority of M.2 are x4 connectors, in fact I'm not sure there's a physical connector defined for >x4 lanes, M.2 is a physical specification definition for a device capable of both SATA and PCI NVMe drives, some motherboards will not support NVMe at all, some will so to some extent and some explicitly support it - it depends on the system board specification, your BIOS needs to support it - just having UEFI does not ensure NVMe is supported, 'lower risk' than what exactly?

Also your statement about PCIe v3 working in any v2 slot is incorrect, I'm sure one or two may but that's the exception not the rule, the opposite is true though, v2 adapters will almost always work in v3 slots.