We have tested the 18TB Seagate and it worked just fine (After the Kickstarter Campaign was posted). So we indicated it in the packaging specs.
Yes, different HDDs require different power and the 2 middle HDD Bays are the only slots that can handle a 3.5 Inch HDD. The two slots on the sides can only power 2.5 Inch.
We don’t expect a lot of users though to use the 18TB at the moment as it is still crazy expensive.