Yes, I'll almost always use a rubber 5 x 3 for the first flights of a plane like this. I'm only worried about the fin being too small.
I had one scratch design launch OK but it "fish-tailed"...it just needed a wee bit more fin.
Some designs would just do a flat 180 at launch and "pancake in".
There is an invisible line that cant be crossed and I'm hoping that this plane is sleek enough up front in the side view to not need much help. It would be nice to have a simple fixture that could cradle the plane and allow it to pitch and yaw while safely suspended. All I can visualize involves impaling the plane at it's global [plan view] CG and suspending it on string inside an air handler unit.
That might test yaw stability, but it would impede pitch movement. For pitch, the plane would need to be skewered at the side view's CG [like corn on the cob holders] and allowed to swim in a bath of moving air to test pitch stability.
IIRC, the most thrust I ever measured from a TD was 16 ozs with a 5.7 x 3. I made a test stand that was pretty close to a miniature "Universal Gym" bench press station and had the engines "helicopter" weights off the floor. By tapping the slide guided engine mount I ensured that the engine was getting full credit for max effort. The only casualty was smoking a new AP .061 because the forward end of the crank ran dry after prolonged vertical running with no real break in.