That is a very real possibility that a lot of HAL pilots (not just WB FOs) could get a bump on their seniority (which could equate to some kind of formula based on DOH or different method). I'm a good example of that. I'm about 33% on the seniority list at an airline that is 60% widebodies (assuming all 20 787s... 54% widebodies if you just look at the aircraft on property now). On a combined list, based strictly on date of hire, I'd be right around 34%... But only about 12% of our aircraft are widebodies. If I don't get some kind of protection (either via fences or a seniority boost), and I were to get displaced because they move my fleet somewhere else, it is unlikely that I would ever get back into a widebody, or at least not for a long time.
So it just depends on the method (or combination of methods) used, and how much they mitigate the potential for harm. In my case, where I already hold the left seat of a widebody, the harm would be actual so there would (in theory) need to be more of a protection in place. For somebody who is 90% on the list and currently a 717 FO, the potential harm is slightly more theoretical as they can't currently hold a widebody Captain seat. But maybe they could hold the FO seat (or could soon), so there is (under the Career Expectations portion) some need to provide a level of protection to them as well.
This is the ugly part of mergers, especially ones that are unbalanced in one of the three legs of the stool.