Hey you forgot perfect 5th - star wars theme
The only problem I've heard about doing this is that you also get the tonality with those intervals in the context of a melody. A "There's a Place for Us" m7 is sol-fa I think, but I think that sounds very different than a Do-te m7.
As long as the interval is in the first two notes of the mnemonic (and not the third or fourth or others one) and you think in term of "Moveable Do" there shouldn't be any problem
Danny there is a problem, you could be mentally changing key with every interval.Take the descending melody G F E D C over a C chord. It is simple to hear this as scale degrees 5 4 3 2 1 in the key of C. However imagine taking the mnemonic approach:Starting on G you sing down to F using the first two notes of Mary Had A Little Lamb - you have just sung the interval in the context of in Eb major not C major! (To continue the melody of Mary Had A Little Lamb would be G F Eb F G G G F F F etc.)Now to sing from F to E you use the first two notes of Fur Elise - you have sung the interval in the context of Bb minor.From E to D you use Mary Had A Little Lamb this time in the context of C major.Finally from D to C you again use Mary Had A Little Lamb but this time in the context of Bb major.Why wouldn't you simply sing major scale degrees 5 4 3 2 1 instead of mentally changing key 4 times with 4 intervals?