The length of the line segment $P\_1P\_2$ is equal to the $y$-coordinate of point $P\_2$, which is the value of $\sin x$. The value of $x$ can be found by solving the equation $6\cos x=9\tan x$, which simplifies to $2\cos^2 x=3\sin x$. Since $x \in (0, \frac{\pi}{2})$, we have $\sin^2 x + \cos^2 x = 1$, which leads to $2\sin^2 x + 3\sin x - 2 = 0$. Solving this equation gives $\sin x = \frac{1}{2}$ (we discard the solution $\sin x = -2$ as it does not meet the conditions of the problem).

Therefore, the length of the line segment $P\_1P\_2$ is $\boxed{\sin x = \frac{1}{2}}$.

By solving $6\cos x=9\tan x$, we can find the value of $x$, which allows us to find the $x$-coordinate of point $P$. Finding the length of $P\_1P\_2$ is then equivalent to finding the value of $\sin x$, which leads us to the answer. This problem tests understanding of the graphs and values of trigonometric functions, computational skills, and the ability to combine geometric and algebraic reasoning.