Hello and welcome to AA!
Most people on here suggest (myself included) to not try and mess with the PH. Using PH Down drops will fluctuate the PH and stress the fish, possibly leading to the death of the Guppies.
I would suggest researching fish from Australia, they will be more accustomed to the higher PH.
In your 70L tank, I would suggest looking into Desert Gobies. They are pretty good for smaller tanks. They like rocks, so I would suggest using sand and rocks for a more natural feel. They are incredibly hardy fish, capable of thriving in temps of 106 all the way down to about 50 degrees F IIRC. Amazingly, they are also found in freshwater, all the way to marine conditions! In your tank size, I would suggest getting about 5-6 Gobies. Here is a good page about them. Chlamydogobius eremius – Desert Goby (Gobius eremius) — Seriously Fish
In the 300L, I would suggest doing many types of Rainbow Fish. They are also from Australia, and come in many different colors, shapes, and sizes. They can be a little more expensive than your average Guppy, but much cooler! There are different species of Rainbow Fish, so I would suggest researching them online, and reporting back with the ones you like.
To answer your question, plants and fish will not change the PH.
Also, I would read these links!!
Articles to Help you Get Started with your Aquarium
Most important is the cycling one.