I'll explain it. I may not be able to make you understand it all at once but, basically, the idea, as expressed in your posts and you plan, that you need to train slow twitch and fast twitch motor units seperatey (with separate rep ranges) is erroneous.
If you read through the link I provided, which is a sample from the Essentials of Strength Training And Conditioning (a book I recommend everyone own) you see a couple of passages that basically explain it very nicely:
"Before a high-threshold motor unit is recruited, all of the motor units below it are recruited sequentially. Thus, with heavy resistance training, all muscle fibers get larger, because for the most part they are all recruited to produce higher levels of force with heavier weights. Motor units
high in the recruitment order are used primarily for high force, speed, or power production."
OK, that part is the bread and butter of it. That is also called "orderly recruitment". The terms LOW THRESHOLD and HIGH THRESHOLD can basically be viewed as slow twitch or fast twitch. Slow twitch being "low threshold" and fast twitch being "high threshold".
"With heavy resistance training, all muscle fibers get larger because they are all recruited in consecutive order by their size to produce high levels of force. In
advanced lifters, the central nervous system might adapt by allowing these athletes to recruit some motor units not in consecutive order, recruiting larger ones first to help with greater production of power or speed in a movement"
The last part about exceptions to the size principle we really don't need to be concerned about.
For the most part, when it comes to resistance training and growth you are looking at those Type 2A fibers. Some of this is still theory and we don't know everything but to keep it simple let's assume that is pretty much covering it.
There may be some growth in Type 1 fibers although there is also some evidence that they tend to shrink with hypertrophy training. See where it says "decease in type 1 fibers" in my post above. Those are the slow twitch.
Going up to 20 reps is getting into endurance territory. Look at the two DIFFERENT adaptations between enduarance training and resistance training.
Anything up to 12 reps (and sometimes as much as 15 for certain supportive movements) is hypertrophy training. All motor units will be recruited from lowest to highest threshold according to the size principle.
Is this making sense yet?
Anyway if you use a lot of these terms for a search you should be able to find lots of into to round it all out. And be sure to read the glossary sticky.