2. Great way to fuck up the glass. Use a dedicated sponge instead, something like a filter sponge. Don't want to get your hands wet and keeping fishes at the same time? Better buy a dog. Always wash your hands before coming in contact with the aquarium water.
3. Most of them don't have a clue. Don't bother asking people working for a big chain store, do your own research.
5. Depends on the breed. Most cyprinidae are hardy and can take it. Depending on the breed, most poeciliidae are also hardy enough. There's nothing stopping you from using a mix of cold and warm water to prevent it from being too cold/warm.. Personally, I use about ten liters of filtered water (to prevent algea growth because our water has a high amount of silicates in it) and normal tap water, I usually change about 1/4 of the water.
6. It's better to put the fish with the store water into a bucket, then slowly add aquarium water to it during the next half an hour or so, then use a net to add the fish to the tank, never put that dirty ass store water into your own tank.
I never had problems with my PH value, I don't know what you're doing wrong. Decorations (don't buy any of that tacky shit, like a shipwreck or other trash, they look shitty, are possibly painted with lead paints and the fish don't care anyway) are cheap: add some stones, some wood and buy some plants. Don't buy colored sand either, use natural white ones. If you aren't completely incompetent, you'll only have to buy plants once. You don't need chemicals or "cleaning supplies" (whatever that means, never use any sort of household cleaners to clean anything in your tank) either. Only the beginning is expensive, after that the costs are more than resonable. I'd recommend buying some fertilizer though.
