EI doesn't address any ratio at all. It merely recommends a dosing schedule that maintains adequate levels of the needed nutrients. Ratios of nutrients have not been shown to be significant for plant growth.

GH has no effect on pH, with or without CO2. KH, which in ideal water is carbonates, does affect pH.

Plants do need some magnesium, so if your water is virtually magnesium free you should add some magnesium as a nutrient.