What is the (grammatical or syntax) term for adjectives you can use them in two ways, with 'not' or without 'not' to say the same thing (more or less). For example: 'not-good' instead of 'bad'. Also 'not-beautiful' instead of 'ugly', and so on.
I think you mean litotes. "Not bad" is a classic example.
Off hand, I can't think of an adjective that can't be negated with the word "not". I don't think there's a special term for that. However, you can't equate "not beautiful" with "ugly". A thing can be neither beautiful nor ugly. It may be just plain, so you could say "Its not beautiful." without implying that it's ugly.