Does every control surface need to have a counterweight?
Flutter is an energy storage and release phenomenon.  Surface moves, applies a load to the fixed surface it's attached to, fixed surface bends under the load, stores some of the energy like a spring, then releases it in moving back to its original position, and some of this released energy is transferred to the moving surface attached to it, which is also stored and released back into the fixed surface in a way that adds more total energy than the first time, magnifying the original movement, repeat, and away you go.

For the this to happen the moving surface has to be unbalanced with a fair amount of mass away from the hinge line, the fixed surface has to be flexible (spring like), the whole system has to have a natural frequency that is close to the frequency of the natural oscillation frequency of the moving surface, and there has to be enough energy present in the first place to get the whole thing going.

Generally airplanes that go less than say 80-100 mph don't have the energy to induce flutter if there is reasonable stiffness in fixed surface like the horizontal stab.  Light airplanes with fabric covered wire braced tails have stabilizers that are quite stiff due to the wire bracing, and the combination of high natural frequency resulting from the rigid system and the low input energy at the speeds they fly at means they don't really need a balanced surface (look at a Champ's unbalanced elevator; the C of G of the surface is probably at 30% of chord - no big deal because it simply can't excite sympathetic movement in the stabilizer it's attached to at the energy level present at its operating speed range).

If the fixed surface is not so stiff, the moving surface is really heavy, or you want go faster, then you start having to consider mass balancing.  To really know for sure what the threshold is, you'd have to have someone that knows what they are doing do the analysis. But generally low speed light airplanes have the inherent rigidity to be immune from flutter in their speed range without balancing control surfaces.

You still have to confirm it in the real world though.  When you test fly your creation, you do some rudimentary flutter testing by diving to a speed a bit above your Vne (actually creeping up to it in a series of steps), pull up so you are decelerating, and as you pass the speed of your test point you slap the control stick sharply, trying to induce an oscillation.  By doing it while decelerating, the risks of an oscillation starting and building into full fledged flutter are low. But, if you see an oscillation that starts but then goes away, well you've "tickled the tiger's tail" and you'll need to balance the surface or reduce the airplane's speed range.  You're wearing a parachute while doing this of course (or whomever you're able to talk into doing your test flying).