Many here will recommend against flake food because it can fall to the bottom and start to break down so quickly before fish find it; personally, I use it (spirulina flakes), and have not had too big of a problem because I put just enough in where the fish eat it all well before the bottom of the tank.
A QT tank will need a filter, a heater, a light, and something for the fish to hide in/behind. Whether you want substrate is up to you, many people will recommend against substrate in a QT for simplicity.
A good vaccuum is where you dig the siphon deep into the substrate and get all the garbage that may be hiding in it. I assume you have crushed coral? Push the siphon as deep as you can go into it, to the bottom if you can, allow any cloud of buildup to siphon up and out, and then move over to the next spot.
As far as the skimmer, I'll let someone else answer that..I'm not sure if a 30 gallon without corals would need a skimmer, but a more experienced reefkeeper could give you a better answer...
38 Gallon (Freshwater): 1 Green Swordtail, 3 Yo-Yo Loaches, 1 Clown Pleco, 1 Blue Gourami, 4 Otocinclus, 7 Cherry Barbs.
46 Gallon (Saltwater): 60 pounds Lalo Live Rock, 60 pounds aragonite sand, 1 Coral Beauty Angelfish, 1 Peppermint Shrimp, 1 Mushroom Coral, 1 Emerald Crab, various snails and hermit crabs.
10 Gallon (Saltwater quarantine):