If you're assigned CRJ you will be eligible to fly both for the company.
You will be trained primarily on the 900 however flying a 900 and flying a 700 is like flying the same aircraft and requires no extra training. The only place a pilot will ever touch a 700 at mesa is if the pilot is assigned to the Washington Dulles (IAD) domicile. New hires nowadays tend to do a short stint there before their seniority allows them to be able to transfer out to another base.

If a pilot is on reserve and flies the CRJ he could virtually fly both variations in a day. A reserve pilot is utilized wherever the company needs are and if a Dulles reserve is needed in Dallas then he could end up flying both 700/900 easily.

In October/November I'm assuming CRJ pilots will be able to bid to Dallas Fort Worth(DFW). This base will be all under the American Eagle Banner and will be solely CRJ900s.

Mesa Bases
IAD - CRJ700 (United)
CLT - CRJ900 (US/AA)
PHX - CRJ900 (US/AA)
IAH - E175 (United)
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DFW- Starting NOV. CRJ900 (US/AA) flying transferred from CLT
LAX - Current CRJ900 operation (US/AA) staffed out of PHX.