Two points which come to my mind. There are many Mashaykh within Tableeghi Jamaat who do Tazkiyah within the framework of Tableegh The present day methodology of Islah adopted by many Mashaykh is also just a form (of Islam) e.g. this method recommended by our dear noble and respected Shaykh (Maulana) Saleem Dhorat is “A METHOD OF TAZKIYAH” is just a FORM. So the methods adopted by our Mashaykh (Naqshabndi-Mujaddidi, Chishti etc) are FORMS while Tableegh is also a FORM and there are other FORMS e.g. Jihad, studying and accompanying Ulama/Mashaykh etc. Tazkiyah is FARD but its forms DIFFER based on Ijtehaad of Ulama-e-Kiraam and a person can pick and choose or combine as he/she sees of benefit. And off course some Ulama/Mashaykh can prefer one FORM over another but that's also down to their personal choice and Ijtehaad.