Rashi: They are appointed over those appointed [over workers].

Rashi: Of the 550, 300 were converts appointed over the 70,000 carriers and 80,000 who quarry. It says (5:29) that there were 3,300 taskmasters, aside from Sarei ha'Nitzavim. In Divrei ha'Yamim II, 2:17 it says that there were 3,600 taskmasters! The other 300 are mentioned here; they were appointed over all of them. The other 250 were Yisraelim. In Divrei ha'Yamim II, 8:10 the 250 are counted by themselves.

Radak: Of the 550, 300 were converts appointed over the 3,300 converts who worked (5:29) ¹ . In Divrei ha'Yamim II, 2:17 it counts 3,600, for it includes the 300 converts appointed over them. There were 250 Yisraelim appointed over the 300 converts; our verse counts both of them.

Malbim (here, and 5:30): There were 3,000 appointed over the 150,000 workers - each was over 50. Another 300 were appointed over 500 each. Here it counts both of them. Divrei ha'Yamim counts also 300, each of whom was appointed over 10 of the 3000 appointed over 50 workers. Shlomo counted 150,000 who converted after David counted the converts. There were also 25,000 from before David; there were 500 appointed over 50 each, and another 50 over 500 each; in all, there were 550. Divrei ha'Yamim II, 8:10 does not mention Sarei ha'Nitzavim. Rather, it discusses taskmasters over the 10,000 Yisraelim working in Levanon. There were 200 over 50 workers each, 20 over 500 each, 20 over the 200 taskmasters, and 10 over 1000 workers each; in all, there were 250.

'Rashi', Radak (both on Divrei ha'Yamim II, 8:10): There were 250 native Yisrael Sarei ha'Nitzavim, appointed over 300 converts who were appointed over the 3,300 taskmasters. Divrei ha'Yamim counts the 300 together with the taskmasters; Sefer Melachim counts them with the Sarei ha'Nitzavim.