Mesa is adding 50 more aircraft? We have 6 more 900's this year and twenty something more EMB's. Anything after that is just rumor. It's thought that we'll get more EMB's for United, but that is just rumor and not hard facts.
IAD is junior because no one wants to be there, thus the junior pilots in seniority can hold captain there long before holding captain in Phoenix (no senior FO in Phoenix would want to be a junior CA in IAD). A junior captain in IAD would only be able to hold FO in Phoenix.