Its legal to take these options together. But what is their actual result?
Unarmed Combat Mastery: A ninja who selects this trick deals damage with her unarmed strikes as if she were a monk of her ninja level –4. If the ninja has levels in monk, this ability stacks with monk levels to determine how much damage she can do with her unarmed strikes. A ninja must have the Improved Unarmed Strike feat before taking this trick.
So without monastic legacy feat we have effective monk level of 10.
Add half the levels you have in classes other than monk to your monk level to determine your effective monk level for your base unarmed strike damage. This feat does not make levels in classes other than monk count toward any other monk class features.
By RAW, Monastic Legacy seems to add 1/2 "non-monk" levels. This can be taken in one of 2 ways:
A) As OP, just add 1/2 of the ninja levels i.e. 5. Which would give the total of lvl 15 monks unarmed strike damage at lvl 14.
B) Unarmed Combat Mastery counts 6 of the ninja levels as monk levels. so the only additional "non-monk" levels would be the 4 remaining ninja levels. Which would provide 2 extra levels from Monastic Legacy for a total of lvl 12 monk smackage at lvl 14.
I think A is more correct though. And besides its quite an investment for not much gain. Thoughts?
I would agree that since the Ninja already gains a benefit similar to Monastic Legacy that with the Unarmed Combat Mastery trick that the character should only gain the 4 levels treated as monk levels since he/she already took a talent that grants him his ninja levels -4 to count as monk levels for his Unarmed Strike damage. That said, as worded, technically this could be a legal combination to use, but I'd ask your GM (or yourself if you're the GM and don't want your game to be abused and think this would count) if they would think this would stack.