according to forgotten realms wiki it is appropriate for Githyanki to train in magic alongside martial arts and I guess they would know. My Lae'zel is a fighter wizard multi and she rocks.
That makes sense but in my headcanon, origin characters belong to the subclass that first appears. So Lae'zel would be Battle Master to me. (And giving her temper, I can see her bossing around and giving orders).
But your combination, although a bit contradicted to my taste, can very well be powerful. That's the good thing about this game, all classes are strong if you know how to use them.
