All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Our answer to your questions will be summarized in the following points:
1- The ruling of praying behind an innovator or someone who has polytheistic beliefs; we have already issued Fatwa 82989 in this regard, and Fatwa 82654 about praying behind a Tableeghi Imaam, and Fatwa 83718 about praying behind someone who imitates a given school of jurisprudence, so please refer to them.
2- We draw your attention to the seriousness of judging a given individual as a non-Muslim just because he has committed an act of disbelief or that he committed an innovation. Indeed there are some conditions for considering someone as a non-Muslim. Indeed not everyone who commits an act of disbelief is a disbeliever and thus the prayer behind him is void and it becomes impermissible to eat the meat of animals slaughtered by him. For more benefit, please refer to Fatwa 91677.
3- The groups which you mentioned are not the same; for instance the Bralwi group commit many acts by which they associate partners to Allaah which other groups do not commit. As regards the Shaafi'ee group, then they follow one of the schools of Ahlus-Sunnah Wal-Jamaa'ah and it is wrong to consider them like innovators such as the Bralwi group. For more benefit, please refer to Fatwa 84248 about the Fiqh groups and the date of their establishment.
4- If it is permissible to pray behind a person, then it is permissible to eat the meat of the animals slaughtered by him. Rather, it is also permissible to eat the meat of animals slaughtered by a person of the people of the Book, whether a Jew or a Christian, if he slaughters the animal according to their religious rites, but it is not valid to offer the prayer behind him. So, one should not doubt the meat of animals and that are slaughtered by the Muslims that are sold in markets on the grounds that many of them are innovators or that they commit acts by which they associate partners with Allaah. Nonetheless, if it is confirmed that someone apostated, then it is not permissible to eat the meat of the animals slaughtered by him.
5- Greeting innovators and other dissolute people with Salaam; if someone commits an innovation which takes him out of the fold of Islam, then it is not permissible to greet him with Salaam.
It is an obligation to cut the relationship with the innovators who call to their innovation if their innovation is considered as dissoluteness, and it is even said that it is an obligation to cut the relationship with them whether or not they call to their innovation.
Ibn Al-Muflih from the Hanbali school said: "Ahmad said: ''It is an obligation to cut the relationship with the innovators whose innovation is considered an act of disbelief or dissoluteness, and cut the relationship with those who call to such an innovation. This obligation is for those who are unable to refute such innovations or fear to be lured or harmed by them [the innovators], otherwise it is not an obligation. It is even said that cutting the relationship with the innovators is absolutely an obligation even if one can refute their innovations and does not fear to be effected by them." This is what is understood from the statement of Imaam Ahmad .
For more benefit, please refer to Fatwa 90026.
Allaah Knows best.