You should really provide sources for your "advice". I have never heard such things and I think it is safe to say that the second is plainly incorrect, and I can create chords from the major scale that sound like crap. Also, the closer two notes are the worse they will sound when played together, though I see that you are discussing consecutive notes in item 1. However, you then ask about chords (a little confusing), so I am focusing on that aspect.
On another note the question presumes an objective measure of "sounding nice" and that is subjective. Though on average most people agree with certain conventions regarding consonant and dissonant intervals.
To understand the phenomenon better requires delving a bit into physics and psycho acoustics. Related to the ratio of the frequencies of the two notes is a tower of harmonics and those harmonics can interfere even when the fundamentals are far apart. The ratio of the b5 to the 1 would be smaller than 5 to 1 yet considerably more dissonant, and a minor second is extremely dissonant. The key to understanding this is to look at whether or not the harmonics line up. In the case of the minor second the fundamentals are close and that creates beating in the combined waveform. Larger intervals can exhibit the same beating, for example a major 7th (an inverted minor 2nd) due to the proximity of other frequencies present. In the case of M7 the fundamental of the 7th is a half step away from the harmonic of the Root note in the interval. So, for all intents and purposes there are 1/2 steps in the spectrum and the ear hears it.
Even if you try to kill these harmonics the ear will create them because it is a non-linear system. This phenomenon is called aural harmonics. This essentially makes it impossible to hear a "pure tone". Our bodies are biologically designed to experience the harmonics sequence, f_n = n*f_1.
The intervals of M3 and P5 have many harmonics aligned and those that are not aligned are far enough apart to avoid the beating that contributes to dissonance. These intervals are generally considered consonant.
I would recommend looking at a text like "Physics and the Sound of Music" by Rigden. It covers all these topics and does not assume a science background.
When it comes to building chords there is more that choosing notes, the order of the notes contributes to the overall quality of sound and there are some rules of thumb in this regard. For example it is generally a good idea to have large intervals in the bass and smaller intervals in the upper voices. The reason for this has to do with the fact that the human ear has limited resolution for distinguishing notes and this resolution is a function of average frequency of the interval. It turns out that out judgement of "consonant" and "dissonant" are frequency dependent. In the bass register a 3rd can sound muddy and is sometimes judged as dissonant, in contrast in the soprano register an M2 can sound consonant.
As for micro tonal chords? I have been interested in such things too, more specifically micro tonal resolutions in chord movement. I use 1/4 tones frequently but it is usually in passing. The 12TET system actually does not adhere to the harmonic structure that our ears and brain are tuned to and there are more that 12 distinct tones in an octave that can be derived from harmonics, I've read about Just scales that have 17 notes in them. In such cases one can manipulate the intervals to play with the harmonics and perhaps create novel (or rediscover old) chords. However, if you are looking at micro-tone scales built up from equal temperament, like 24 tone chromatics etc. I think these will always produce more dissonant intervals than the standard scales we use. In the case of the 12TET major scale (i.e. diatonic scale built from tones in the 12TET chromatic scale) the difference is extremely small and most folks cannot tell that the harmonics are slightly off. In my opinion introducing more micro tones hurts in this regard more than it helps but that's just my opinion. Some of us enjoy the beating (pun intended) and dissonance becomes enjoyable. Other than the description of consonance and dissonance in terms of harmonics I have not seen a general rule of thumb for chord building that would generalise to micro tonal scales.
