The only real caveat with using the solar CTs is that in order to calibrate you’ll need to get 500+watts on the generator inputs.
I would be inclined to just run the transfer switch feed (from the generator) through the Sense main CTs anyway. As I said, you won’t have issues of “double” power … you just need to make sure to run the wires in the right direction through the clamps. That way any load (the panel circuit breakers that are left on when you are in transfer mode) will be clocked by Sense in the same way it’s clocking the Mains.