1) How many IAL's should I use and how much will they affect the pH?
As many as you want. There is no hard set rule on how many to use. They may or may not affect pH.
2) One of my LFS's suggested using peat moss but the product they had only lasts a month and I'd need to buy at least 6 boxes for it have any impact on the water given the size of the tank. Is there any product I can use to naturally lower pH? And if there is, where's the best place to buy it from? (Don't want to use chemicals bc it's bad for the shrimp and the phosphates would encourage algae growth.) RO/DI water or rain water. There's also alder cones.
Anything that may naturally lower the pH (re: leaves, cones, peat) will also result in tea colored water or "black water".
This would mean that it would be best to have at least one 55 gallon drum (208 liters) to store water in prior to doing water changes and either toss in a pump or an airstone to circulate the water, and toss in a bag of peat/cones/leaves to get the drum water to be the same as the tank water.
3) My RO water goes into a 1000 ltr water tank for when I need it for water changes. Is it worth dropping in IALs into the water tank (in a bag of some sort) to condition the water before use? If so, how many should I use and for how long?
Have you tested the pH of the water after it's sat in this tank? Sometimes, allowing water to sit out overnight and aerating it can allow the pH to drop on it's own.
If you were to try going the "black water" route, you would probably need a lot of leaves, cones and/or peat...
4) Any other suggestions?
What is the GH, KH and TDS of the tank?
What are those same parameters for the water going into the tank? (you mentioned RO - does it come out at 0 GH and KH? And low TDS?)