1)
Is Adoniram the same as Adoram that David appointed over the tax (Shmuel II, 20:24)?
1.
Radak: Yes. He is also mentioned with Rechav'am (12:18) with the name Adoram 1 .
1
Mahari Kara (12:18): At the time of greatness, in the days of Shlomo, he was called Adoniram.
2)
What was the tax?
1.
Rashi (6, 13): It was 30,000 men working for the king (5:27). Mishpat ha'Meluchah authorizes him to collect it (Shmuel I, Perek 8).