Yes, we should see the CRJ-900 flying for US Airways Express in the near future.
After they receive about 3 more CRJ-701s, all further deliveries to PSA should be the CRJ-900. (So yes, US has plenty of orders/options for the type.) Of course, that assumes that there will be further deliveries to PSA after the 3 CRJ-701s. If PSA does not take any more planes, or perhaps even if they do, Air Wisconsin's contract already provides for them to fly the CRJ-900 as US Airways Express.
I'm the expert on here on two things, neither of which I care about much anymore.