Those Enneagram guesses for Frodo and Sam sound about right.
Merry did seem to me to be a thinking type - he handles everything actually pretty calmly and I don't think ever has a very big emotional reaction. ESTJ could be possible, I suppose. Hobbits in general seem to be very S, and he came across as N in comparison with them.
Gandalf always seemed pretty F to me. Why did you think INTJ?
Yeah, I didn't really have a clue with Legolas. I nearly put him as S because he does seem to be S comnpared to other elves...just not in comparison to everyone else.