If you stick with the different flavors within the same brand you "should" be okay, but nothing is etched in stone. I think the biggest problem with gas and loose stools is people feeding too much. Premium foods are rich, and higher in calories. Less is better.
As for the salmon, I guess that depends on where the salmon comes from? Not sure on that, but I do know it's not uncommon for dogs to not like fish. Some kibbles are made up of human grade meats, so I don't see mercury levels being an issue in these brands, unless, as you say the mercury is high in all salmon.