JayC has this covered!
Cherry shrimp I would aim for a bit higher TDS, 180-200 though.
You will be ok with fish as the tank is so heavily planted the fish won't get all the babies but guppies explore every part of a tank so they will get a good few new born shrimp. The flip side to that is you shouldn't get over-run with shrimps? I used to have endlers with my cherries in a densely planted tank, obviously the more guppies you have the more the shrimp babies are at risk? The 8 you have now should be fine but best to decide on a limit early on?
Yes, as long as the rock in in the tank, it will release carbonates, unless the pH goes above 7.0. But that means your shrimp will be living in sub par parameters. If you keep the rock in there, you will constantly be battling TDS, and the constant fluctuation of TDS going up and down is what will stress out your shrimp.
Go by the TDS meter. Much easier than measuring GH/KH.
It depends on your bio filtration. If you have a massive overkill external filter with lots of bio media, maybe 20 guppies in 30L.
If you don't, no more than 10 in 30L of water.
Most likely from the farms.
The Oregano oil is not like cooking oil. It should mix in with the water enough to combat the bacteria.
If you have air bubbles or a filter outlet breaking the surface, that is probably the most ideal.