About superlative adjectives

James asked about when should we add "-est" behind the adjective and when add "most" in front of it when it comes to a superlative adjective.I think as a general rule, we need to add most in front of the long words and -est behind the short words. It can be very vague to talk about short and long.But for a word of only 1 syllable, I have only seen people use -est.For example, fattest, biggest, highest, tallest,etc. In the meantime, I have only senn people use most for words of 3 or more syllables. We always say most beautiful,most wonderful, most important,etc.But for words of 2 syllables, I've seen both. I;m not extremely sure that both apply to the words of 2 syllables, but I think it should be. If you find something that I make a mistake about, please tell me.
                                                                  By Luo