I am not sure, I haven't used peat or almond leaves(though I have a pack) but you should start probably putting a little bit to see how much it will make a difference, and then increase or dicrease the ammount depending. It all depends on your particular type of water so it will be trial and error the first time. Start slow and then work it up till you get the desired result. As for almond leaves, I love tannins, wish the driftwood was still leaking some
What type of corys do you have? Most prefer the water on the soft side though home grown ones would be ok with harder water. Mine are in 7.4 with no problems.
As far as I am aware, lowering the Ph in terms of adding CO2 for example or certain off the shelf chemicals is no good because it doesn't change the mineral content of the water, which is what you need. I don't know anything about those "pillows in the filter" but you need something natural that changes the mineral content of the water like peat or almond leaves. Of course RO water will do the trick but is a lot more work I'd imagine and additional ammount of money.
As for the guppies, your water is still on the high side even for them. I'd add the almond leaves regardless of which way you go, because fish that are used of hard water will become succeptible to diseases in softer water and those are supposed to have natural antibacterial/antiseptic, etc.. properties though this is not exactly proven.