1. All of a sudden a funny thing happened.
2. All of sudden a funny thing happened.
----------------------
Can we omit 'a' before 'sudden'?
3. detour
(Do we stress the first syllable? Can we stress the second syllable?)
4. I have little money.
5. I have less money than Tom.
6. I have the least money of all my classmates.
(Are they all grammatical?)
No, "a" belongs there. 1 is correct; 2 is incorrect.
3 -- stress on the first syllable, not the second
4 - 6 -- Yes, all are grammatically correct.