I have no qualms about using the expression "I live in a country town", until proven otherwise. To me, it conveys the idea of "cidade pequena/cidade de interior/cidade do campo" etc.
And I found it in the English-speaking mainstream media, take The Guardian, for example:Travel tips: cultural Somerset and the week's best deals ...
http://www.theguardian.com › Travel › Somerset
20 de jul de 2014 - You're viewing the Guardian's new website. ... On the surface, Bruton is a fairly typical West Country town – handsome, peaceful, unassuming.
As for county town
, I found that it can be a major city in a region, etc. Perhaps is more of a British English definition.
In the States it seems like a little neighbourhood that is "undecided" on which district to locate, so it put feet on both or border two districts etc.http://www.co.greene.pa.us/secured/gc2/ ... cation.htm
Pennsylvania Route 88 — ... this highway runs through small boroughs such as Carmichaels and Rices Landing during its stretch through the county, before exiting on a bridge over the Monongahela to reach its terminus in the small Fayette County town of Point Marion
Before, Fayete was a county-town, now I see references to it as a county (condado), same goes for Sommerville. Which seems to indicate that the "county-town" concept of old, has fallen in disuse in the States.