Question about AsRock X399 Taichi

Just take one of the more expensive full modular power supplies then it is surely compatible, I myself have a Corsair HX750i together with this mainboard, I can surely recommend it :)

It is best to choose a power supply that has 2x 4+4 pin 12V connectors. You can either combine the 4+4 pin connectors into one 8 pin connector, or just plug one half into a 4 pin connector. It always depends on the mainboard, with the 4+4 pin you are more flexible. So yes, 8 pin on the mainboard is (almost) the same as 4+4 pin on the power supply.

By the way, most mainboards also work if not all additional power connections are used. Nevertheless, you shouldn't do that, because other problems can arise and the plugs get hotter because more current flows through them.