Hi,
I don't know the technicality of power head size...Kiefers is very good at sort of thing, so follow his advice.Two gives you better control to be sure that the water is circulated throughout the tank.
Don't feel too bad about the pet store, I have three around me, and none of them carry good equipment. I purchase almost everything on-line except fish food. Country folks get rude when you make them feel dumb or uneducated. When you go into a fish store knowing more than they do, because of information you got on a fancy computer...they are going to feel offended and act rude. It's best to just not tell them where you got your information or that you are new at everything. Make it sound like you were sent to the store for a certain thing and stay with that.
My late husband was was a good ol boy from Tennessee, right on the border of Kentucky, petstores down there cater to farmers and more into horse feed and dog food.
If you have small children, use egg crate to cover the tank and remove the hood. Egg crate can be found at any Home depot it is in the lighting department, I think it's actually called diffuser, if you ask for egg crate they look at you funny. You can also invest in a canopy, it makes the tank taller and not so easy for little fingers to reach into the tank. I think it makes the tank look nicer as well.
You won't kill your rock or sand not having a power head yet. Get them ASAP to circulate the water....you need lots more rock about 1/2 way up the tank, and it needs to be under the sand so push all the sand to one side add the rock and push the sand back into place around the rock. Make sure your rocks kind of lock into each other and is very strudy. A rock slide could kill your critters or break the tank.
I woud get at least 1 T5HO light, 10K bulb with one actinic...the fish lights that come with the hoods don't do a saltwater tank justice, the fish are so colorful and the better light will bring that out. I have the linkable T5HO they are inexpensive and are cool to the touch. It's easy to upgrade if you decide to get coral later. I use 1 over my 56g tank, and I have the 3 linked up over my 90g tank.
I still don't understand your filters...are they canister...hang on the back overflow like a Penquin or whisper style or a wet/dry sump...???? All these have carbon..LOL, what type do you have. Oh and the strip tests are fine for cycling the tank, you will need the lab type before you start adding fish.