Question

Why is the mad bad'l mentioned as part of the medd far'ee (secondary medd)  and not the basic medd (tabee'ee) ?   

Answer

The secondary group of lengthening (mudood, which is the plural of medd--lengthening) is the lengthening due to either a hamzah or a sukoon, and they can be lengthened more than two vowel counts.  The key word here is can be lengthened.  This does not mean they have to have more than two vowel counts.  Medd badl occurs when there is medd letter preceded by a hamzah.  This is different than the other mudood in that the affect on the medd comes from a letter preceding the medd letter and not after.  

Hafs ‘an ‘Aasim recites the medd badl with two counts, but Imam Warsh ‘an Naafi’  recites the medd badl 2, 4, or 6 counts.  Since the recitation of Imam Warsh allows this medd to be more than two counts plus the fact that the medd is due to a hamzah, this medd is in the secondary medd group.