Peat add acids to the water, eating away the carbonate system, HCO3-, CO32-, and OH-. lowering the (KH) buffering capacity of your water. Here, the ph will probably go down.
The molecules of the carbonate system that largely attribute to this ability to buffer large drops in pH are HCO3-, CO32-, and OH-. These molecules are bases and when H+ (acid) is added to the water source, the H+ ions will chemically bond with the bases. For example:
OH- + H+ —-> H2O;
CO32- + H+ —-> HCO3-;
HCO3- + H+ —-> H2CO3
it is shown here how the H+ ions chemically bond with the carbonate minerals. When the only carbonate mineral in solution is H2CO3, all the alkalinity has been used up since there are no carbonate molecules left for the H+ ions to chemically bond with.
You can add peat to your filter. The only concern I have is if this filter is your main biofilter (Nitrifying bacteria prefer ph around 7)
For the substrate, I see lot of people using peat in all kind of variation and others saying that it can cause trouble.
Personally I think that adding a small amountl of peat moss to the substrate can be positive. The purpose of peat moss is to add some organic matter to the substrate to provide a kick-start mechanism. In a freshly set up aquarium there is no organic matter, so the plants cannot get all the required nutrients right away. In an established tank however, things are different as the substrate accumulates enough organic matter to keep the plants going. Peat can help the plants kick in straight away.
In order for iron to be available from a substrate, you need to use a clay, soil or iron containing substrate additive together with a small amount of organic material such as peat. The organic material provides nutrients for anaerobic bacteria to reduce insoluble iron (ferric) to soluble iron (ferrous). It also releases humic acids which are natural chelator chemicals which lock onto positively charged chemical ions like Fe++ and make it available in the water. These humic acids also help to buffer the pH in your aquarium to a good value.