I agree with the others recommending that a quarantine tank will be needed, and not just because the new fish might bring something in, but because your old fish may have been exposed to something along the way, something they are now immune (or built up a tolerance to) something the new fish may have not tolerance for (ick can be included in that scenario). And if the new fish become infected, that infection might then come back to overwhelm the tolerance your old fish.
Now for my opinion of the peat...its usefulness is not only the perceived changes in water chemistry (whether real or not) but the tannins produced by peat, act as a natural bactericide for fish endemic to tannin infused waters. These tannins might not be enough to stop an acute infection, but may be needed over time to fight off chronic infections like HITH, or other normally non-threatening infections, but those that become problematic for fish endemic to tannic waters when kept without the tannins.
Now for my opinion of the peat...its usefulness is not only the perceived changes in water chemistry (whether real or not) but the tannins produced by peat, act as a natural bactericide for fish endemic to tannin infused waters. These tannins might not be enough to stop an acute infection, but may be needed over time to fight off chronic infections like HITH, or other normally non-threatening infections, but those that become problematic for fish endemic to tannic waters when kept without the tannins.