There is certainly room for a third non-com in a large market. But you'll have to be tough. You can expect the NPR station to suck up at least 90% of the available underwriting money and a big chunk of listener support.
Go to the Radio Research Consortium site for some very helpful reports done for public radio stations:
http://www.rrconline.org/reports/
These are probably more general than the format-specific info you want, but you'll get a ton of info that will help you make your programming more popular with the people who would pay your bills. I know that I learned a lot from reading them.
For one thing, you really have to have specific programs that people happily pay for to keep them on the air. NPR knows this and airs specific shows that will bring in the pledges. Morning Edition and Car Talk bring in people who will pay to keep hearing their favorite show. Not just ratings, but pledges and underwriters.
A mixed music format will most likely not generate this kind of listener loyalty. People just don't seem to dig into their jeans for a few bucks to keep music on the radio. But they will pay to keep various specialty shows on, if they can't get them anywhere else. It's hard to compete with most MP3 collections, so a successful station is going to be talking about things that fascinate the audience, rather than playing tunes that are readily available.
You might do well to affiliate with Pacifica, which is very inexpensive to free. They have several shows that are crowd pleasers, particularly Democracy Now and Free Speech Radio News. Those are potential fund raisers with a built in audience.
So if you're going for a music format, set aside those "hard to find an announcer for" morning drive hours for news and info programs. Your first fund drive will tell you what the listeners are willing to pay for.
Whatever you do, make sure the station develops a special identity that people recognize you for around town.
NPR stations have very high ratings. They are generally in the Top 10 in most markets where they have a signal comparable to the commercial stations. It's hard to go head to head against that. Better to be the nimble little guy who can broadcast the shows that aren't heard anywhere else in town.