Yup. That's the goal. If you see ammonia or nitrite at any level besides 0 (and I mean even a tinge of color that isn't dead on the color it is supposed to be), I would do a PWC. Maybe not a huge amount if the color is just a tinge on the wrong side of 0, but still enough to remove some of the toxins and replace with fresh water. It may not seem like a big deal, but how would you like to be swimming in your own feces? I am one of those people who feels that it is better to change the water too often, rather than not often enough. Only once did I come across a problem with too frequent of water changes... I got a bit of black brush algae, which came as a result of inconsistent CO2 levels in the water. I believe the true cause was insufficient circulation in the water more than excessive water changes. I increased the circulation by adding a second filter (I had been planning to do that all along, I was just waiting for it to be shipped) and I didn't change the water at all for 3 days. The stuff just disappeared. The folks in the planted section said that I would need to actually remove it manually, but that the other steps would stop it from increasing. Well, in my case, it just up and disintegrated into nothing!
Ultimately, I think that you are closing in on the end of the time. If you go a full 7 days with double zeros across the board, regardless of what your pH is, you are cycled for your current bioload. I would wait a full 14 days before I considered adding any more fish, and in your situation with the pH still being a bit unstable, I'd wait until that fully stabilizes. The crushed coral will do the trick, but it will take a little while before the buffering capacity of your water comes back into the position it needs to be.
One word of caution: You are using crushed coral to increase the kH of your tank. Always be aware of that. The reason most folks don't do things like that is so that a big water change is always available as a basic cure-all solution to nearly every problem. You will not have that luxury. (Of course, you really didn't have much other alternative.) You will need to be a bit more careful about advice folks give you on this board. The last thing you are going to want is to make matters worse during a crisis. (Doing massive water changes could actually do more harm than good in your situation, given the potential difference between your tap and tank water.)
As far as routine maintenance goes, you are going to have to find the rhythm with your tank. The pH will probably always drop a little between water changes. You will need to find just the right time and the right amount of water to change so that you don't shock your fish with too big a change in pH or kH. I would suggest that you change your water when the pH drops no more than 0.5 on the scale. It is going to take you some time to figure out how frequently and how much water you need to change so that you can keep the pH/kH fluctuations small. For example: If you currently have 7.6 pH, change it when the pH drops to 7.1. But, if you change only 30% of the water, it may not go back up to 7.6. Then, you could change it again when it goes to 7.1, but this time change 40% of the water to see if that brings it back to 7.6. It will take some time to get into the rhythm you need. My guess is that in your situation, more frequent but smaller volume changes would probably be the way to go. Personally, I do a 30% water change every week to 10 days. (if it is 10 days, I change 50%). But, my water is locked in at 6.8 pH and my tap water is 7.0. So it really doesn't matter much for me, my tank and tap water conditions are almost identical (except for the amount of minerals and nitrates). So, a big change for me doesn't cause an issue and is a perfect cure all. For your situation, you might need to change your water every 3 or 4 days, but only about 15% of the volume. This will ultimately have about the same effect as my changes, but with smaller fluctuations.
