The 777/787 are a pretty long wait right now, in fact, most (not all) of the junior FOs on those fleets could hold narrow body CA in one place or another, and some of the senior FOs on widebody fleets could also hold CA on those same fleets . The most junior 777 FO that I'm seeing is just below 11,000 in the company, so 4000ish numbers from the bottom. That will obviously start to trend down as retirements start to increase. The 787 is more senior than the 777.
The 767 is much more junior, in MIA we sent new hires into it for a good chunk of 2014. It's still fairly junior, but not new hire junior these days, some mid-2015 hires are on it in MIA and some mid-late 2014 hires in PHL.