res = TraditionalForm[
HoldForm[Subscript[W, f[a, b]][
c] == \[Rho]^f[a, b] \[Omega][c] \[Rho]^-f[a, b]]];
res /. {c -> 7, f[a, b] -> 5};
For your second question you can try this:
g[a_,b_]:=a Sin[b];
res /. {c -> 7, f[a, b] -> g[2 , 3]}