I wouldn't think the size of the ethnic population in a particular market matters as much as trying to get people to actually listen and ultimately, finding enough advertisers who are willing to spend the amount of cash on the station that's required to make it viable financially.
To build on your post, while I'm aware of a number of stations that offer an hour of foreign language programming here and there (English language stations that offer a block of ethnic programming daily, etc.), or stations that offer ethnic music programming at certain times during the weekends (some brokered and some done in-house) are there many independent full-time ethnic and foreign language stations, or are most of those stations owned and operated by a larger broadcasting company, with most all the same programming distributed to all the stations they own?