"men, wise and known": Of the seven (qualifying) traits mentioned by Yithro to Moses, he found only three: "men," "wise," and "known." "and I made them heads over you": heads (i.e., granted precedence) in buying, in selling, in (business) dealings, in entering and in leaving — entering first and leaving last. Thus, "and I made them heads."
"officers of thousands": If there were 1,999 less one, then he (the officer of the 1,000) was made the officer (of the additional 999). "officers of hundreds": If there were (an additional) ninety-nine, he (the officer of the 100), was made their officer. And thus for the officers of fifty and the officers of ten.