Adding 20 more pounds of rock ? :bounce: NOW your cookin' with gas!! Good choice!!
The nassarius snails will usually only come out when you feed the tank. They have an excellent sense of smell. My favorite snails. If you look closely, you can see their little "elephant trunk" sticking out of the sand. They rarely go completely submerged where that "trunk" isn't sticking up just a little. Mine come out at night occasionally. So cool to watch them burst from the sand bed as soon as they smell food. Actually pretty fast, for a snail. I had 2 in my 30g reef, but recently added one more. Mine are the size of a grape.
I picked up an SPS frag when my tank was only like 2 months old. It's now about 6 months old. SPS frag is doing very well. I have ZERO nitrates in my system though. I know nitrates are bad for SPS, but I don't know if they can tolerate a little bit. I think that if you honestly have ZERO nitrates and you know WHY you have ZERO and you can keep doing that--you could try a small SPS frag. I attribute my success to a DSB in the tank and the refugium, as well as a massive chunk of cheato in the refugium and 61lbs of rock in a 30g tank. Yeah, it's packed!
I've always wanted a school of fishies. If I had at least 75g, I might try a school of blue chromis. Like 5 of them, maybe? With your 55g tank, you might get away with 3 of them. Another really cool fish that I like it the red stripe goby. They are small--only grow to about 1" and they live in little schools, so you can keep several. To me--you could easily keep 3, 4 or even 5 of them in a 55g and they wouldn't really add squat to your bioload if you get your rocks up to around 100 pounds.
You could also think about a dwarf angel. A Flame angel or a Coral Beauty or maybe a Lemonpeel Angel. They are all "sorta" reef safe. You are taking a chance by adding one to a reef. Some of them will nip corals--some won't. They are so beautiful--worth the risk IMO. I've been hunting for one locally. No joy yet.
"Cougar Bait" LOL Where do ya get one of those? OH wait! Nevermind, I'm already married. :grumble: