Takahashi Ai joined Mini Moni and the 'rule' about the height was changed. Also, if I'm not mistaken, Takahashi had actually said that Ayu could join Mini Moni because of her height which I thought she mentioned was 153 cm (i'll have to rewatch that episode!). I'm thinking Ayumi is just over 5' and official reports have her as a bit taller than she is.