Tashahhud is an obligatory part of Prayers we are been ordered to perform Prayers exactly as the Prophet (SAWA) performed صلوا كما رأيتموني أصلي
We are not allowed to add on obligatory Tashahhud any word.
Tashahhud contains mentioning Imam Ali (AS) and all the Infallible Ahlul Bayt (AS) in saying ALLAHUMMA SALLI ALA MUHAMMADIN WA AALI MUHAMMAD.
It is allowed to recite any Du'a during any part of Prayer (Salah) so it is allowed to say ALLAHUMMA SALLI ALA ALIYYIN AMEERIL MO'MINEN or any supplication like that as Du'a which is not part of Tashahhud.
Our greatest scholars the Maraaje' of Taqleed have said that any deliberate adding to obligatory Tashahhud invalidated the Salah.
I advise you if you are still in doubt to seek the Fatwa of the Marje' of Taqleed.