The state I mostly live in has 254 counties and tons of cities, so that'd be a chore.
My personal take. Texas needs more categories than just the five listed, although I guess you could consolidate some of these to meet the original criteria. I cut off the Small Town from Rural at at about 20K - the smallest Small Town here is Brownwood in Brown County. If you've ever been to the Rio Grande Valley, you'd never be able to describe it as truly "urban" so an Urban-Suburban definition is more applicable; the distinction between Urban-Suburban and Suburban-Exurban is kinda used by whether the county is accessible by a looped highway or merely a straight highway. A little arbitrary overall, perhaps, but it makes sense to me. 
