Here are the specs for the Ryzen 5 2400G: http://www.cpu-world.com/CPUs/Zen/AMD-Ryzen%205%202400G.html
According to CPU World you can configure the 2400G to run at a TDP of 45 Watts:
Here are the specs for the Ryzen 5 2400GE: http://www.cpu-world.com/CPUs/Zen/AMD-Ryzen%205%202400GE.html
The 2400G has a TDP of 65 Watts while the 2400GE has a TDP of 35 Watts. They both have the same AMD Vega 11 Integrated Graphics.
You can use Ryzen Master to Over/Under Clock your Ryzen 2400G Processor or you can use your motherboard's BIOS if they have the settings for it.
As for the best Mini-ITX motherboard, There are many that supports Ryzen 2xxx Processors. You need to go to the motherboard that you are interested in and check to see what features it has or open a Support Ticket with the motherboard's Manufacturer.