Yes, I think you're doing a good job and getting better and better at learning this stuff but no, I don't think I'm too confused but instead feel there's still some info that's not been put across to you in a detailed enough manner yet. It could very well be that you lost most of those fish mostly because of an uncycled tank, rather than disease, or at least because infections or conditions they had were worsened enough by the bad water conditions that they were unable to make it. Its quite possible to have platies and, to a lesser extent, guppies, have a higher survival rate in an uncycled tank than some of the other fish you mentioned in the threads.
Just taking a look at your last post there.. excellent that you used tank water if washing the filter. Doing 25% water changes per week during fish-in cycling might or might not have been correct, one can't tell as the real goal during fish-in cycling is to never have the ammonia or nitrite rise above 0.25ppm before one can change the water again, otherwise you're left not knowing what amount of damage might have been done to the fish. Filter supplement would not have hurt anything but would not have really done anything either. Denitrol, not sure whether that would have been harmess or negative.
OK, enough of the "sounding negative" stuff, sorry! It sounds like you are not monitoring the right things and each day you continue to get "double-zeros" (meaning on ammonia and nitrite) the more confident you can be that your two fledgling colonies of bacteria will not have too serious a mini-cycle when a few more fish are added. Just be aware that your colonies are bound to not be very "robust" yet, so be sure to keep any new fish introductions pretty small, I'd say.
~~waterdrop~~