3. Your experiment is leading you in the correct direction.
The guideline you are looking for is this: Use the lowest numbered ISO that still allows you to get the shot.
If you are shooting a static landscape using a tripod, then by all means use the camera's lowest ISO 100/200, whatever that is.
If you are shooting moving subjects in low light, then you need to push up the ISO to whatever numbers are necessary to get the shot. Better a noisy pic than a blurry pic since this noise can be worked with during post processing.
As you have started to notice, the use of the picture will help dictate how much noise is acceptable. Pics that are scaled down to web viewing or 4x6 prints hide any noise very well, 8x10 prints will need some work on them.