I'd strongly recommend buying yourself a liquid test kit. Not only does it help you to accurately determine right now what your water is, but also to monitor it in the tank over time (it will deviate from the source water) its also a very important tool while cycling your tank to detect Ammonia and Nitrate and Nitrite (all at a given rate toxic to fish).
Given the pH by your water company, this let's assume you have harder water, which would be great to set up for Cichlids. This pH however kicks quite some Tetra off the list; so I'd suggest to go the Cichlid route.
For water exchanges its recommended (dep on exact stock) to do anywhere from 30-40% weekly every week. Your filter will sort out depbris to some extrent, but your filter will not remove what the fish leave behind daily if you know what I mean
A good dechlorinater mixed in your tap water to remove chlorine and metals will be just fine.
If you still plan on adding plants to the tank, I'd then suggest plants with heavy root systems to be added to a cichlid tank (Hygrophilia, Swords Anubias