You will need to remove the covering where the tri stock will contact the stab and fin. Things don't stick well to covering films and even if they, the weak link would be the bond between the covering and the underlying balsa.

I don't think any streamlining is probably required as the square joint between the stab and fin already has tons of drag all by itself, but I tend to taper the front and rear anyway for a better looks.

I also cover the 45 degree edge of the tri stock with monokote for looks and for fuel proofing. I will wrap it around the edges just enough to avoid any bare wood showing on the exterior.