You're approach to fixing that should work just fine. You will never notice any difference in the way the plane flies.

Another alternative would be to glue a piece of balsa (1/8" wide or so) to the back edge of the forward fin which would in essence allow you to make it taller (but also wider). However, from your picture, it looks like the assembly is already wider than the plans show, so just sanding down the front edge to get a nice smooth line from base to tip is probably certainly easiest. If you want to get fancy, you could add the piece to the back of the forward fin and then sand the forward fin to match the plans and then add it to the fin. If so, try to use balsa that is the same hardness as the existing fin/forward fin.