The first thing is that I wouldn't trust the test results from dip stick tests. Those are all known for being very inaccurate and I have seen many people wipe out their entire tanks by using & trusting those things. You can't know what needs to be done unless you have accurate results. Is there any way to get ammonia, nitrite, nitrate, and pH readings from liquid test kits? Can your lfs test it with liquid kits and write down exact numbers for you? Or maybe a friend who has aquariums and liquid test kits could help you out?
I am still not understanding what the organic waste digester is, can you send me a link to the product online so I can see what it is, review the contents of it, etc? There should be no reason to add that kind of product if you are doing proper maintenance. There are a good number of those kinds of products on the market and most of them are very misunderstood. One example would be Easy Balance by Tetra. When that product first came out people began using it in place of water changes... which is not how it should be used and can crash a tank quickly if done so.
In answer to your question about high nitrates and fry, yes, high nitrates can cause issues for spawning any species of fish. A sick fish is not likely to reproduce, and poor water quality will kill newborn fish quickly.
Can you please explain to me the 2 - 3 minute and penny size rule? I am not familiar with that one.
In regards to salinity... with the species of fish you listed, salt is not something I would be suggesting you use regularly. All of the species you listed in the 30 gallon are sensitive to salt. If you can read salinity on a hydrometer or refractometer, that is way too much salt for most freshwater fishes. Salt can be used for medicinal purposes for the guppies if needed, (at this time I see no need) but I would not be dosing any salt into the 30 gallon.
It sounds to me like you have a lfs that is good with their sales techniques and at taking your money. I would strongly suggest you let their suggestions go in one ear and out the other from this point on. If they start to suggest chemical products to you, your first question to them should be "what does it do?" and your 2nd question should be "why do I need this?" Then, read the bottles! The majority of lfs employees know very little to nothing about the products they suggest to you. Their #1 priority is to take as much of your money as they can. A tank with issues will bring you back more often to spend money if you "trust" them to guide you through it, and unfortunately, too many people get caught in that kind of trap.
I will wait to find out about this organic waste digester (product and company name, a link to it online would be most helpful) and some accurate test results. Once we have that info to work with, then we can make some progress.
In the mean time, 25 - 30% water change each week, gravel vac every 3rd change. That is standard maintenance on most aquariums that are not over populated. Increasing the water changes to this will help, and slowing down feedings to once/day will help too.