Tragedy... Not sure what to do.

Its normal. But, there are so many factors that go into the nitrogen cycle. For one, the water you used, if it was tap water then it has chlorine. And a dechlorinator is needed. Tap water has poisonous stuff in it that kills beneficial bacteria and fish, lead, copper, etc.

Second, water changes. During a cycle the bacteria bloom and over crowd, causing many to also die. This raises ammonia, which is bad. The bacteria take ammonia and start to "fix" it into less lethal forms of nitrogen (then plants use it to grow, producing oxygen). But, with too many bacteria forming the cycle is broken and cant balance its self at first. The fishless cycle takes longer because the Nitrogen cycle needs waste to start, a fish cycle takes less time but is very stressful on the fish. I'd recommend leaving those fish as they are, feed sparingly. Do 5% water changes daily, and get a declorinator. Jungle Start Right/Safe Start (available at wal*mart) wouldn't be bad to get either. It provides a boost of good stuff to establish the bacteria colony much faster.

New water should be placed into a bucket thats never been used with cleaners, buy a new one for tank use only. Pour some of the declorinator into the taer and allow 10 min to pass, then pour this water into the tank.

 

I -doubt- it, I've left town and forgot to turn my lights off, but my tank was established. As for the "bad fish" I'd say thats vary likely. Most pet stores get there stock from fish mills (sorta like puppy mills, only less heart breaking), and the stress they under go in shipping is immense.

For the time being, do small, daily water changes and be sure they aren't overfed. Go on and leave the lights on, since they'll be turned back on any way. Get some of that start right stuff I mentioned, it's not expensive and it really does save time and money.
Oh, and only treat the new water, and treat the water before it get put into the tank. I'm not sure if it would hurt to treat the tank water or not, but it might.

Also, are there any white spots on the fish? Are they slow moving, or showing any torn-looking fins? Whats the temperature in the tank?

And be sure that they aren't over feeding the fish

 

I'd test the water (or have your local fish store do it for you and write down the readings for future reference), it sounds like your tank is still cycling which is odd considering you used SafeStart (I had great luck with that and TLC Smart Start as well).
1) I'd get a good dechlorinator such as Seachem Prime (which is also a good baddy remover).
2) Siphon on a regular basis from the bottom of the tank (cleaning the gravel too)...then BEFORE adding the clean water dose for the entire tank volume with the Prime. (it's good stuff, so you'll only need a little). Smaller tanks will need more frequent cleaning ime, so I'd say about 30-40% weekly.
3) Feed a good quality food because it will make your fish more vibrant, healthy, and help you to maintain clearer water. I love frozen Hikari Bio-pure bloodworms because my fish don't leave any behind and feeding is as simple as popping a cube into the tank. There's also alot of high quality flake/pellet foods on the market...New Life Spectrum quickly comes to mind. And only feed an "eyes" worth...if you're afraid it's not enough, then up the feedings to 3x's a day. You don't want to see food hitting the bottom of the tank (being wasted)...you want to see the fish coming to the top and basically scarfing the food before it has a chance to float to the bottom.
4) Keep your water temperature consistent and monitor it with a good thermometer (digital probe...I believe the tom temp alert reads within +-1 degree). With livebearers you may want to keep your temperature a bit warmer..78F - 80F
5) Be mindful that with females you will more than likely end up with multiple babies...which can quickly lead to overstocking. Overstocking WILL cause water chemistry issues. Most with molly's will tell you to add "freshwater" aquarium salt, however you'll want to avoid that with guppies.
6) I know you mentioned about lighting being left on too long, that would not cause death (algae, but definately not death). You can always plug your light fixture into a timer to avoid people repeatedly turning on/off your lighting. I wouldn't suggest the lighting being on more than 12hours.

 

If you've already added it to your tank initially you shouldn't need to continue adding more.

If you check amazon, you'll see some really good reviews..when I used it I was in a hurry and didn't get a chance to do water tests which would help to prove any effectiveness. I did the simple pour the bottle in and add the fish and didn't experience any negative effects and surely didn't experience a single loss.
http://www.amazon.com/Tetra-77961-SafeStart-30-Gallon-3-38-Ounce/dp/B002DZG1BW


Also, if know someone that has an aquarium setup and running (or are lucky enough to have a local fish store that would be willing)...you could always give your cycle a kick in the pants by adding a "used" sponge filter, handful of gravel, "used" media... etc from an up and running tank which would contain benef. bacteria.

Old Way Of Cycling:

Dealing with New Tank Syndrome, Cycling and Nitrogen Control

(day by day cycle chart) --> Expectations of a Typical New Tank / Aquarium Cycle Process

 

Good luck =D

 

Water changes should be done according to tests, you need to be keeping your ammonia as close to 0 as possible. The fish are dying due to ammonia levels, water changes will be the only way to keep them from continuing to die.

 

when first establishing a new tank, fill it, set it up, and start it. add 1 tspn of sea salt to start bacteria...NO Charcoal at this time. run it for 1-2 weeks empty, then toss a goldfish in there. What your after is bacteria establishing in the tank. This bacteria had both good and negative effects on the bio-cycle of the tank, in this case, you need it. Let the goldfish feed and live a happy 7-10 days. Do a partial water change. 1-2 gallons at the most. This adds freecycle in to the tanks cycling and now your ready for fish. Tap water is okay if it is untreated or NON-city water. If city water, treat a 5 gallon pail of water with a small pump and airstone for 24 hours prior to the water change. Add 4ml of StartRight. This simple chemical neutralizes the ammonia/copper/diodes in the water. Be sure to check for high ammonia levels during frequent intervals. If you have Well water, no treatment is needed. Well water is naturally hard, and iron based supplements. To remove hardness from the water (this is simple) add pretreated malasian driftwood, which accents your tank nicely, and lowers the pH level. It also absorbs trace elements helping keep your tank balanced. If your intent on keeping african cichlids, no wood is needed, they need very hard water. The harder the water, the better. If South American Cichlids, add as much driftwood as your heart desires, they thrive in neutral to soft acidic water.

Alot of folks believe that your water has to be 4500% perfect for your fish to thrive...most fish come from petshops, and they maintain their water far worse than a beginner hobbyist. I have vile tested a few petshops and found acid in the .08 range, amonia in the .7ppm range, and some was far worse.

 

any progress?