I would like to ask if there is a specific way of Positioning of legs while sitting in Tashahud or in between the Sujood in namaaz ?

Is it Wajib to position legs in a particular way ?

what if one does otherwise? eg. keeping left ankle on top of right or just sitting on both the legs or just sits down and legs aside?

What is wajib is to sit on both legs. Yes there is a mustahab way but the method you described on placing one leg on another is ok