Don't do it. Rabbits are worthless and not worth risking a long lived treasure like a tortoise. Rabbit may want to chew the legs. of the tortoise. Get a cage for the crapping king of the habitat and make him live in his own feces.
Don't know how cold it is at your location. Live on the coast, never gets too cold. Have no heating lamp. Mine get cold, as they do in nature. Warm them up in the morning with a warm bath or UV
Tommy my Russian has been with me for 10 years plus. Peroidically, winter or Summer, he takes as much as 7 days to sleep whatever it ia off. Do not know about thunder storms. Likely normal behavior