I've heard that as my bike (Mongoose Maurice 2012) came fitted with an 18t freewheel (large diameter thread) the smallest it can take is either 15t or 16t. I had intended to fit a 13t freewheel; but I've heard that a 13t freewheel has a smaller diameter thread designed especially for a BMX cassette and so a 13t will not fit on the cassette of the Mongoose Maurice 2012. Is this correct?

THANKS