Can we add ashadunna ameeral momineena and Ali Un Wali ullah in tashahud of wajib namaz

No but according to Ayatollah Sayed Sadiq Shirazi you can add as a mustahab but needs to be worded specifically in a certain way which is different than what you mentioned. He says you can say wa anna ‘Aliyan ni’mat wali. For more see his islamic law book