Bismillahi Al-Rahmani Al-Rahim

 Praying Behind Followers Of Other Imams
al-salamu `alaykum

Concerning the issue of Hanbalis praying behind non-Hanbalis, Nail al-ma'arib gives the following
If the imam omits [either] an integral or condition that there is disagreement concerning (m: whether it is an integral or not, or whether it is a condition or not) because of [making] taqlid (t: of an Imam) his prayer is valid. (t: But if he did not omit it out of taqlid then the imam and whoever followed him must repeat the prayer.)
And anyone who behind him (t: behind anyone who has omitted an integral or condition) while believing that his prayer is invalid must repeat it.
(t: In the Iqna` and its commentary it says
And if the imam knowingly omits [something that] he alone holds or that he and the followers hold [to be] an integral, obligatory, or a condition, then they both repeat it. But if it is something that the follower alone believes [to be an integral, obligatory, or a condition]--such as a Hanbali following someone who
(1) touched their penis [with their palm or the backside of their hand --musa]
(2) neglected to cover one of their shoulders or becoming motionless during prostration [tum'aninah], or the like
(3) [omits] saying Allahu akbar when moving [from position to position]
out of making taqlid of someone who does not consider these to invalidate [the prayer]--then neither the imam or the follower needs to repeat the prayer. And similarly, if a Shafi`i were to pray before the [mosque's] assigned imam does so, it is valid for a Hanbali to pray behind him.
End of quote [from the Iqna`])
It is not permissible to rebuke matters [based on] ijtihad (t: meaning the issues for which there is no evidence that clearly must be acted according to [laysa fiha dalil yajib al-`aml bihi wujuban zahiran], such as a rigorously authenticated hadith that is not opposed by another hadith of the same category.)
(Dalil al-talib with commentary from Nail al-ma`arib, 1.178)
R eaders of Sheikh Nuh Keller's translation of Muhammad Naqib al-Misri's Shafi`i class Umdat al-salik wa `uddat al-nasik now know the details for Sheikh Nuh `Ali Salman's commentary
(f12.29) It is valid for a Shafi`i to follow the leadership of an imam who follows a different school of jurisprudence whenever the follower is not certain that the imam has omitted an obligatory element of the prayer, though if certain the imam has omitted one, it is not valid to follow him. The validity is based solely on the belief of the follower as to whether or not something obligatory has been omitted.
(N: One should mention the position of the Malikis and Hanbalis here, which is that the criterion for the validity of following the imam is the imam's school of jurisprudence, such that if his prayer is valid in his own school, it is permissible to follow him as imam. How close this is to the spirit of the Law, which strives for Muslim unity.)
(The Reliance Of The Traveller, f12.29)
A nd Allah knows best.

wa al-salamu `alaykum
--musa