How did the election of one-third of Senators every two years stay in sync with addition of new states?
score:5
Accepted answer
so were there some Senators who took truncated terms?
Yes, that's exactly how it was done. For example
Vermont elected its first two Senators in 1791. One Senator was up for re-election in 1794 (and lost), and the other was up for re-election in 1796 (and resigned).
Hawaii elected two Senators upon its statehood in 1959. One term expired in 1962, and the other in 1964.