How difficult would it be to switch substrates from inert to the ADA soil? Also is it possible to do this with shrimp already in the tank?
If I was doing a 20gal or maybe even a 30gal I would consider plant specifc substrate. However doing a 75-90-110 I wouldn't consider it, at 20$ a bag and atleast 2 - 3 inch substate will cost more than my regulator EASILY.
What is high tech about it?
If it is low light then I wouldn't worry about special substrates, the light will be your limiting factor. It is important to maintain balance with all the components of plants (light, CO2, ferts, substrate, temp, etc.). If it is out of balance that means you have excess of something, which will be used by something (usually some sort of algae).
it can increase ammonia a lot
It does do that...It is recommended for a newly set up tank with nothing in it except plants, until the Ammonia is lowered.