First off, don't trust the info you read in a book. Usually it's fine, but sometimes it can get corrupted (Stocking plans, fish sizes are the most common I find).
Secondly, Petsmarts have a reputation for being really bad. The ones I've seen are worse than I thought they'd be. Hopefully you have one of the better ones, but I'd suggest getting fish somewhere else.
Now, this one is really important. You have to 'cycle' your tank before you stock it. There are 2 ways: Fishless (easy) and Fishy (cruel and hard). Before I get on to those, I will tell you how the cycle goes.
At first, you will just get readings of ammonia and nitrite, prefferably over .5 ppm (usually higher, which can be achieved by 'feeding' the tank more). After it's been like this for a while (it can take days for it to have readings), nitrates will start to show. This might take a few weeks. Once the ammonia and nitrites have gone down quite low (far down below .5 ppm) or even disappeared, do a large water change. Test and change until you have 0 ammonia, 0 nitrite, and 5-15 nitrate. Fish can typically take up to 40 nitrate, but it's unhealthy for them. Now, get a few of the small fish you'll stock, and wait a week or 2. Get some more, repeat until you have a full tank (this usually happens 3-4 times).
You can either use Fishless, which lets you 'feed' the tank, or Fishy, which is putting a hardy fish in and letting it's gills be ruined and shortening it's life, and letting it crap instead of feeding. To keep the fish alive, you'll be testing once to twice daily and changing the water every day or 2. Financially, Fishless is better as well.
Now, a few random tips:
-Treat your water with Prime or Stresscoat (Prime above all others). They are dechlorinaters and provide helpful slime for your fishes skin.
-Check with the members here before you buy any plants, fish, or invertebrates. You may be buying a monster fish.
-Make sure you have all the equipment. Net(s), Filter(s), heater, gravel, tank, hood, lights, etc.
-Do NOT get UGFs (under gravel filters). Black gunk will come out of it and foul your tank eventually, and I don't find them very effective.
-Don't buy less than 6 schooling fish, and 4+ social/grouping fish.
-Don't buy an over-large fish hoping it will stay small. It will either get big and (probably) murder everything else, or it will get stunted and die a bad death.
As for your list, I'll put the bad ones in red, the maybe ones in blue (yellow is too light), and the goodies in green.
Guppies - Good begginer fish, keep at a ratio of 1 male for every 2/3 females, they breed like crazy but usually tankmates and parents eat them. Very hardy, cheap, grow to (males) 1.5" and (females) 1.5-2".
Danios - Good begginer fish, need schools of 6+. Very active, somewhat easy to breed but need a specialised tank to do it. Extremely cheap, tough as nails, and only get to 2" max.
Platies - Good bigginer fish, very similar to the guppy in characteristics. Livebearing and a crazy breeder (like the guppy), they come in many, many color forms (like the guppy). Hardy, cheap, grow to 2-2.5".
Rosy Barbs - These are fairly hardy semi-begginers fish, but they need schools of 6+ and get 4" long, and arn't all that cheap. Can be nippy. Not for your tank.
Gouramis - Depends on what kind of gourami you want. If you want plenty of other fish, sparkling, honey, and dwarf (although they are currently quite frail from inbreeding) are nice. Pearls get pretty big. Three spots (AKA blue, gold, etc.) are pretty agressive and get 4-6". Sparklings get 1.5", honeys 2", and dwarves 2-2.5".
Clown Loaches - Fairly hardy, moderate care levels, IMO. Need schools of 4+, but love to dogpile with more, they get 12-16" and need tanks of 180g+.
Clown Plecos - I'm not sure on this, because I've heard of clowns and royals and others all getting huge, and only one kind only gets a few inches. Anyway, most kinds get 12"+ and are 30$+, so I'd skip them, IMO. They're also crap factories.
Angelfish - These are very pretty fish, but get large (6" long, very tall) and, IMO, need tanks of at least 40g, although some people say a 30g is enough. You could only have 1, because the alternative is 4 (way, WAY too many for a tank under 55g). Delicate as youngsters, fairly hardy as large adults, anything less than 4 will have a week one to be picked on, unless a single one or a breeding pair (sometimes 1 will still kill the other).