I was never one to stock my tank lightly (my reef is busting at the seems with fish right now) but your nitrates are a concern. If they are at 10 ppm, and you have the filtration you describe, your tank may not be able to handle its biological load. You need to get your tank to where the nitrates go down to 0 or undetectable, and then you can start worrying about what and how much to add. There is some formula to follow for # of inches of fish per gallon, but I don't know it since I never followed it. Everyone always falls into the same trap at one time or another, overstocking, and in a small dose, it's okay. However, if you overdo it, you'll start losing fish, and then you'll regret doing it. When I had a 55 FO tank, long long ago, I used between a 3"-8" substrate over a reverse flow undergravel with additional mechanical filtration. The substrate was crushed coral, and I had 0 live rock in it. My occupants at one point were 2 eels about a 1.5' long each, a trigger, a lion fish, 4 damsels, a butterfly, an angel, a blue box fish, and a tang, and probably some more fish in there as well. I was meticulous about water changes (5-10 gallons per week and a 50%+ every month or so when I bleached my corals). I never let the phosphates become detectable, nor the nitrates. I have no doubt my tank was very overstocked for the majority of time I had it, but I never had any problems as I had tons of hiding places to prevent aggression and did a lot of housekeeping to keep the water quality tip top. If you are going to overstock, make sure you're careful to avoid aggression, and also, get your water to premium quality to not stress anyone out. It's a bad thing to overstock, but it's also a very hard thing to avoid when you see something that you've just got to have. I still make the same mistakes today that I made years ago, but I've never killed anything off because of attrition, just improper research on diets (a linneatus tang in my reef that I assumed ate the same thing as other tangs, but which I found out is more of a planktivore - is that a word?).