Using new docks/dongles on old Macs?
The Apple adapter is bi-directional and power pass through works well. You won’t have any issues other than being restricted to Thunderbolt 2 speeds. (Which should be an obvious limitation)

Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter
https://store.apple.com/xc/product/MMEL2AM/A

USB 3.1 to 2.0 works for data and lesser power, so that’s not a full featured as Thunderbolt. Most devices should work fine if they don’t need USB PD.