One thing to keep in mind is you shouldn't have plans to put a "lot" of fish in with corals. More fish, more food, more waste from the excess food and from the poopin fish. Not a good situation for corals. Plus the most beautiful fish tend to potentially nip or outright eat corals.
In my setup I've found some small fish to work very well... purple dotyback, green chromis (x4 b/c it looks kool when they school), royal gamma basslet, 6-line wrasse, mandarin goby dragonette, neon goby, orange diamond goby, might be something else in there? It's a 100 Gal tank. Note... these are all small fish. 1" to 5". So the point I want to make is you're not gonna be adding a whole lot of fish to 42 gal. Now, I also have a couple of other fish... Flame Angel, Maroon Clown. Angel requires more greens for his diet, and a clown may not do well without an aneome. And anybody will tell you here that aneome's are a pain in the ass. They're hard to keep, they keep moving all the time, and they'll harm your other corals. I had a clown swimming in one one time a couple of years ago for a short period of time till something happened.. and since then I've tried MANY times to duplicate it. You gotta make some choices. If you want an aneome then you will have to sacrifice on what else you put in the tank... corals and fish. Aneome's can eat your fish too.
My suggestion... look at those fish I mentioned above. There are many kinds of wrasses and other fish too. The ones I picked are very colorful, easy to find, cheap, reef-safe, non-agressive. Be careful mixing fish of the same species too. You can check out places like
www.marinedepotlive.com and
www.liveaquaria.com to get info on fish and corals. Continuing with my suggestions... I'd stick with those types of small fish that meet my criteria... and look around for a diff. wrasse if you want. There's tons of them. Another kool fish that is good is hawkfish, red type my fav. Problem is they may go after your "shrimp" if you don't keep him fed with ghost or marine
feeder shrimp. And shrimp are kool. You definitely want to have some shrimp. So watch out for fish that eat shrimp.
That's the easy part. Corals are another story. With such a small tank I'd keep it simple. Some corals don't like a lot of light so arrange your rock so you have an overhang or two to put them in. Some require strong light so put them at the top. Hardy corals are ... plates, leathers, gorgonia trees are hot, pulsing xenia, brains, mushrooms, polys (buttons, zooanthids, ricordia). I would take your time and only put one thing in at a time every 1-3 months... waiting for something that looks nice to come along in your local shop. Another reason for this is to allow for your bio-load to adjust and for the coral itself to settle in. Often it's good to put things at the bottom for a week or two and work it up closer to the lights to where you want it to be. Especially true if you have strong lights. It sounds like you will have average lights.
I'd keep up on trace elements too, especially calcium so your coralline algae can grow on your rock. Shrimp need iodine to molt. Maybe you can find an all-in-one additive. You should start off slow with all of this, test your water weekly, constantly adjust your feeding to find the right amount.
