frank wrote:
Wouldn't the spruce fillers act as a lamination on the spar caps?

Yes but only where they are attached. Since "between the ribs" was
mentioned I'd say there was no benefit.

However, if the plywood skin is handling any loads eg. torque loads,
then the balsa may be a problem!!

Balsa is used in the Knight Twister (cantilever biplane). It has a
plywood torque box (probably a better term here), and uses the balsa to
fill in the leading edge and tips.

So yes, balsa (or foam) can be used in some places for lighter
structures. But I'd be wary of using it where it attaches two stronger
materials..

Regards

Martin Morgan