The carbon rods as you have them are fine for wing bending loads but not so much for torsion. Adding foam will not add any torsional strength either, the fiberglass will but of course will add weight, more so than adding balsa. If you plan to use Monokote on top of foam or even foam that has been glassed, think again. Epoxies thermal glass transition temp is right there with Monokote’s application temp.