Definition of RP/poly is not clear!
The circuit $C_n$ has $n+m$ inputs, $x_1,\ldots,x_n,r_1,\ldots,r_m$. Its size is polynomial in $n$, so without loss of generality, $m$ is polynomial in $n$. What we want is:

If $f_n(x) = 0$ then $\Pr_r[C_n(x,r) = 0] = 1$.
If $f_n(x) = 1$ then $\Pr_r[C_n(x,r) = 1] \ge 1/2$.

Here $x$ is shortcut for $x_1,\ldots,x_n$, and $r$ is shortcut for $r_1,\ldots,r_m$.