1 country and 7-10 cities

Hello. Can you help me...

I have only one country, how i can set it as default and hide ?

in this country i will be use only 10 cities. So, can i use region like city ? and, if yes, how i can hide old field city ?
thanks

@Anton.D
That is changing of osclass structure not included in support.
You need to investigate replacing regions by yourself.