Growing pains when adding smaller equipment types...
In about 2 more years, when the 767 FO's start bidding the 747 FO vacancies (3 year seat lock for lateral movements), all new hires will probably start on the "light twin" as it normally is for pay and experience purposes.
As for a little lower pay for two years (first year rate is equal), consider this - where else can you make 50k - 80k your first year? Besides the "big two", where else can you get to triple digits so fast as an FO? How many places posess such a chance for growth / upgrade to Captain pay?
The 767 is a great plane to fly, and already is a great money making platform for the company. If Atlas is right for you, I suggest you happily take a seniority number on any equipment offered.