First off, welcome to AA !!! !!! !!!
It sounds like your starter kit is a pretty good one. All the equipment that came with it sounds good and pretty adequate for the job. I have no personal experience with Whisper filters but it is a reputable brand so I would think it would be fine. In a tank as small as a 29
gal, there is really no need to add a second filter (i.e. an undergravel one). They were in vogue a few years back--probably about the time the book(s) you were reading were published--but overall enthusiasm for them has waned a bit in recent years, for reasons too long & complex to get into here. They still have their proponents, but given the fact you already have a perfectly fine filter that came with your tank setup, I would suggest saving money on buying an ug filter.
Yes, you will need gravel. A good rule of thumb when trying to figure out how much to buy is one pound of gravel per gallon. Most any gravel they sell in a good pet or aquarium store will do. Personally I would shy away from the gravels that are too wild colored, as I think in the end it distracts from the beauty of the fish, but if this is for a 7-year girl old she might very well want the hot pink or electric purple gravel...and who's to say that's not perfect for her.
I've never even heard of a condensation tray, so I really don't think that's anything to worry about.
I wouldn't worry about your water. One thing to consider is that if your pet store is anywhere near where you live, then the water they are putting in their tanks is the same water you are putting in yours. If it works for them, it should work for you.
If you buy a water hardness testing kit (which will test general hardness, gH, and carbonate hardness, kH) and post those numbers here, we can try to suggest fish that would be particularly suited (or unsuited). But in general, most aquarium fish can be very adaptable to wide ranges of pH and hardness, as long as your keep them steady. Having wide fluctuations in pH or hardness (usually caused by well-intentioned but misguided attempts to alter these values through chemicals) is a recipe for a fish disaster.
Speaking of testing kits, you absolutely must get a good freshwater testing kit that will test for pH, ammonia, nitrite, and nitrate. Don't get one that uses paper strips, get one that uses reagents and test tubes. It is absolutely critical to know what is going on in your tank chemically, especially during the first several weeks when the tank is trying to "cycle." Bio-Spira is said to be nothing short of a miracle for "instantly" cycling a tank, assuming you get a good batch. That can sometimes be hard. While many of the people on this forum have had amazing success, I have tried it twice--once was a "partial" success, the other more or less a failure. But considering it only costs around $12, it is definitely a worthwhile investment. Note that you add the Bio-Spira simultaneously with adding the first batch of fish, not before.
As for water changing, the Python is a product people rave about. I haven't yet gotten to the point of being convinced it is worth the cost, so I use a good ol' fashioned syphon and bucket. How much, and how often, you change depends a lot upon the chemistry of your tank; early on if your tank is not yet cycled, ammonia et al. are gonig to be building up and so water changing needs to be more frequent than down the road once your tank is fully cycled. You might get a variety of opinions but in a cycled tank, changing 10-20% once a week is a reasonable expectation. If your tank is lightly stocked, 20% every other week actually might suffice.
Mollies are great fish, especially those dalmation ones, but one thing to be aware of is that mollies like a higher salt concentration than most other freshwater fish...in fact, they like it almost brackish. That is fine if you are interested in having *only* mollies, but if you are wanting other fish as well, it may cause problems. Mollies can survive in regular freshwater, but it is not ideal, and they tend to be more prone to disease. There are a lot of great fish out there. For a 7-year old, I would think fancy guppies would be a good possibility--they are very brightly colored, come with great variation from reds, yellows, blues, spotted, you name it. Platies and swordtails are also quite colorful, and all of these fish (like mollies) are livebearers...which means if you have males & females together and they are alive, they will be making more babies than you will know what to do with. Although the females tend to be a little less striking (especially when dealing with guppies), you should really buy them at a ratio of 2-3 females for every male; if there is just one female and one male, the stress of the male constantly wanting to mate can cause problems for a lone female.
Other colorful fish, which would fit in a community tank of the size you have, would include a school of any of the smaller tetras (neons are beautiful but not the hardiest fish in the world; I would not suggest attempting to cycle a tank with neons), any of the smaller barbs (cherry barbs stay relatively small, do best in a male/female or male/2females group, and the red on the males can be striking), or a single (or a male/female pair) dwarf gourami. Down the road, when you have an established tank and pristine water conditions, you could consider moving to something like German Blue Rams, which
IMO can rival any saltwater fish out there for sheer beauty.
Also, a tank as large as a 29
gal will always benefit from some sort of bottom dweller. I would suggest getting a school of 5-6 cory catfish. They are the cutest little buggers in the world, very energetic, and they will do a great job of cleaning up any food that makes it to the gravel. They are schooling fish so don't get just one or two, as they will not be nearly as happy. And though you might be tempted to "mix & match" (there are ~ 160 varities of corys), they do *best* when the school is all of the same species. They are fabulous and so much fun, and get along with basically any other fish you could put in a community tank. Most get to about 2" adult size; if you can find pygmy cories, they only get to about 1" as adults and are cuter still--if you can find them, a school of 8-10 would mean non-stop antics in the bottom of your tank.
In summary, really the biggest hurdle to setting up a new tank is cycling. 98% of all problems new aquarium owners have are problems that are either directly, or indirectly, a result of poor water chemistry due to the tank not being cycled. Once you are over that hurdle, the sky is the limit.
Feel free to ask any follow-up questions you may have! And good luck!
