get another 50 pounds of live rock and put it in the 75...cycle the tank, then in about a month, move everything over. piece of cake. yes, you will be able to add fish and corals to the 75 if you have enough lighting to support the corals and the fish are the correct species that will thrive in that size tank. don't buy anything that will compete with the mandarin for food (wrasses, copperband butterfly, etc.).