Ich is a parasite, so low ph or any other water issues doesn’t cause ich at all. Poor water conditions could certainly lead to the fish being stressed/unhealthy, which in turn would allow the ich to gain a stronger foothold on the fish.
QT tank size depends on what fish you have, and likely what fish you intend on getting in the future.
I think using the DT water is easier on the fishes acclimation into the QT tank, though there’s certainly folks who prefer using new water. After the initial fill up though you’ll only want to add new water to the tank, no more using anything from/for the man tank in the QT tank.
You’ll want to use a hob filter, and if you’re using copper make sure you don’t have carbon in the filter as it will pull the medication out of the water. A small powerhead/waterpump would certainly be nice to add circulation. Also, test the QT tank water often, and do water changes frequently to make sure the ammonia/nitrite levels don’t get out of hand. Ammonia will kill the fish faster than the ich will.
In the future put all fish into the quarantine tank for several weeks to avoid ich, or other worse problems in the main tank. It’s not too bad for you to have to do it with one fish, if you have many it’d make it far more difficult.
Edit> oh, and no. Ich will only effect you fish, it is posible for the dormant stages of ich to be hanging out on corals/liverock, though they aren't a host for them and they can't sustain the parasite. Inverts, ie snails, shrimp, anemones, are also safe. though putting anything wet through a QT process certainly isn't a bad idea.