Dear Swati,
Since Empl Standing orders Act does not apply to your company ,you are at liberty to fix it differently for different levels of employees. But I don't see any sound logic in fixing different probation period for each employee. You may consider fixing only two periods. For senior level one year and for the rest six months extendible for further period of three months . Purpose of probation period being to assess suitability of each other and too many different periods, in my view, would be unfair and unjust.
Regards
V.L.Nagarkar
HR-Consultant