Sorry about the loss. Yeah try to learn the basic first. And yes this hobby can get very expensive. I lost a lot of fish and money to learn the hobby too. However there are products that can speed up the process of cycling the tank for you.
I posted some recommend products on the Bacterial...
Yes some species can adapt to range of PH if the species are stronge enough to stay long enough to the adaption.
If any of them are weak, they will get sick and die. I did a test on 20 neon on two tanks; tank #1 has PH 7.6 and Tank #2 has PH 6.4; I lost 7/10 Neon on tank#1, and 2/10 on tank#2...
I have bad experince with Guppys too; Not that there was bad water chemistry. It is the breeders who breed them in a mix DNA. New generation of Guppys are getting more fancyer less stronger. White Spot is a sign of stress from the Guppy; they can't cop with the stress.
Hard work! ha. In past, I think I had the same situation that you have. Here are some advise. No need to change water to reduce Ammonia every day. That's what I did in the past when I was in panic. I still lost fish. So here what you need
Biological Additives ( I perfer StartSafe)
Filter...
Tested these three products on my old and new tanks
Safe Start
Bio Spira
stability
Safestart works within a week
Bio Spira works within a week
Stability does not work
I use Safestart more becasue it is cheapper than BioSpira.