This puzzle will necessarily have several possible answers; here is one I came up with. To reach 9 words with only 10 different letters, each word in the list should only introduce one new letter, with the exception of the first word, although even that needs to be limited to just two different letters.
MAMMA MA
MAGMA G
LLAMA L
HALAL H
LAUGH U
GHOUL O
AMIGO I
HUMAN N
LOGIC C