Joining Rakahs of the Tarawih Prayer

Is it permissible to pray a set of rakahs of tarawih with one salam? If so, does this set have to be of even number? Furthermore, if one is permitted to join the rakahs, can one choose to add a testification after all or some pairs of two consecutive rakahs? 

In the Name of Allah, Most Merciful and Compassionate

assalamu `alaykum wa rahmatullah Sidi

In the Shafi`i school, the tarawih prayer is only valid if it is prayed in sets of two rak`as. al-Khatib al-Shirbini (Allah have mercy on him) said in his commentary on the Minhaj, "If [the tarawih prayer] is prayed in a set of four rak`as with a single salam at the end, it will not be valid, because it goes against the way that [the prayer] was legislated." (Mughni al-Muhtaj, 1.346, Dar al-Ma`rifah (Beirut)).

And Allah Most High knows best.

Hamza.