Yeah its OK to do that. I just did that with the 75gallon tank i just set up. I put the LR in about a month ago and let it cure for about 3weeks doing 5gallon water changes every 4to5 days. then about a week ago I put in the substrate bed and let it cycle again for about a week. I only had a minimal ammo spike and everything else was great. I just added my first fish yesterday. If you do it slow and deliberate you shoudn't have any major problems. Plus you can see if yo have a good set up in terms of water flow and all that jazz. It was well worth the wait for the fish. remember slow and steady wins the race. good luck