Question 1. Most tweak guides can be helpful if you understand what you are "tweaking" and how it will affect gameplay. The most important thing is being able to undo the tweaks without reinstalling.I tend to reduce the graphics settings manually before I add or delete any esp from my Data folder, or change anything in my ini settings.
Queston 2. I do not use that tweak guide myself. I tend to look at the readmes and comments for individual mods that I have installed.
Question 3. Many mods require tweaking and configuration to work properly, Streamline is a perfect example. Streamline requires a lot of adjusting to work properly, but many people get annoyed with the program because they fail to read the readme when they installed it.If you look at Load Orders where people are talking about how stable the game is with 200+ mods, You will always see Streamline in the Load Order.