Why not use CitySelect for just the large cities and then a different mapset for everything else. From what I have seen CitySelect eats up about 3x the memory as Topo and Roads and Recreation.
And to answer this question, that would be fine, except with metroguide, topo and all others besides city select, you dont have autorouting...
The base map is suppose to have autorouting. So you can autoroute around a city and find the necessary places and still autoroute between cities just without the special stuff. You also then have your topo maps and what not when you want them.