Several issues in this thread, i will try to answer from my experience/research.
First, Prime is a water conditioner and should only be used as such, i.e., when changing water use it for the amount of water being changed. Prime detoxifies ammonia, nitrite and nitrate; these functions work for about 24 hours, after which, if there is still ammonia/nitrite beyond the capability of the bacteria and live plants, it will be toxic. However, read on before jumping to conclusions.
Prime detoxifies ammonia by changing it to the relatively harmless ammonium. Test kits read ammonium as "ammonia," and bacteria use ammonia/ammonium. Live plants need ammonium, so they will grab a lot of it. Prime detoxifies nitrite by binding it somehow, I do not fully understand the chemistry but that is not important.
I said Prime is effective for about 24 hours. By then, the idea is that the bacteria/plants will have things under control. However, in new tanks it takes time to establish a workable bacteria colony, and the more fish and the smaller the water volume, the riskier this is because more ammonia (and then nitrite) is being produced than the bacteria can handle so soon, and in a small volume of water it becomes more highly concentrated. Thus, if you do a water change with Prime, you are fine for a day. If the next day you find ammonia or nitrite above .25, do a 50% water change with Prime. Keep doing this until ammonia and nitrite are zero. Nitrates are somewhat irrelevant through this period, but when ammonia and nitrite both rise and then fall back to zero you will likely see nitrates, although with live plants they may be minimal.
I spoke to someone at Seachem and they told me to use the Prime every 24-48 hours during cycling. I am doing this to help my fish, even though it isn't much at least it helps. I do plenty of water changes but if I do so many to keep nitrite at 0.25ppm or less, my theory is that the beneficial bacteria will never have a chance to multiply to get rid of the nitrite, which is why I'm using the Prime to reduce the toxicity. I also put an extra air stone in the tank because I've read that the Prime can reduce the oxygen levels.
Bacteria supplements and seeding: Never use gravel from a fish store. "Stuff" can live in the gravel that you do not want in your tanks--pathogens, bacteria, disease, whatever. While some of these may also arrive with fish and plants, or anything else like rock or wood out of a fish tank, there is more likely to be more of this "stuff" in the gravel. There are two bacterial supplements that are safe and they do work. Seachem's Stability [you mentioned this, good one] and Tetra's SafeStart. These are 100% live bacteria. "Cycle" is different, I'm not saying it might not work, but I know the other two do and they are safe.
When I went to the fish store to ask for some gravel the guy there informed me why it wasn't a good idea to do that, which I totally agree with. They wouldn't give anything to anybody to help seed their tank because they think it is too risky. I was just told by some members on here to try and get some but I'm glad I didn't succeed.
Snails are your best friend in an aquarium. They perform cleanup that you could never fully match. Be glad they are there. [I'm talking about the small snails, like pond, bladder and Malaysian Livebearing.]
I know they are a good thing, but in two days I've found about 15 or so. I took out about 10 of them because I don't really want a huge swarm of snails on my aquarium glass. Do they eat the live plants though?
The dead platy: From your description, and the cycling issue, this fish probably died of nitrite poisoning. Nitrite gets in the bloodstream and becomes methemoglobin which turns the blood brownish, as you mentioned at the gills where blood is closest to the surface/water. Methemoglobin renders the blood unable to carry oxygen and the fish cannot get oxygen so it literally suffocates. Fish may appear to "survive" cycling, but in almost all cases internal damage is done, and down the road other health issues may arise, or the fish may (often) die prematurely, "for no reason" externally. The poisoning by ammonia or nitrite is the cause. Nothing can be done now. We all live and learn from our mistakes.
I am aware of all this and knew it was bound to happen once I read about the nitrogen cycle. I feel bad that it was my fault but it's all a learning experience and I definitely won't do it again. They need to educate employees more at Petsmart and think more about the welfare of the fish and not the money, but then again I'm the fool for buying the fish without researching very much. My friend works at Petsmart and told me they can't tell a customer not to buy a certain fish, even if it is not compatible with the fish in their aquarium or if it's way too big for their aquarium.
Hope this has helped to explain things, feel free to ask questions. We are all here to help each other, and we never stop learning.
Byron.