Hello! And welcome to SimplyDiscus! I would definitely spend some time reading our beginners board to get a good understanding of discus.
You have a lot of questions which is great. Id like to suggest giving some thought to more grow out tanks. It sounds like you plan on starting up a small hatchery. If you have even 2 pairs breed at the same time in those 8-29 gallon tanks you will have a real hard time growing out fry with only 2 each 55 gals. Its just not adequate.
On your TdS.. you can still get discus to breed at 150 TDS... you will.usually have higher hatch rates if you get the Tds down which is why RO water is what many here use.
its fine..if anything it can trigger spawning.Is it too hard on the fish to go from 150 tds to 75 during breeding
Starting with 8 each 2.5-3.5 " discus in a 55 gal is ok.. but they will outgrow that fast and its harder to keep up water quality.. 55 gals are really better suited to juvenile discus and if you want them to grow big its going to be challenging.Initial stock is something I'm pretty up in the air on. Worth it to pay double for 3.5s compared to 2.5s. 1 strain in each 55 gallon, start with around 8 per tank,
Al