the 1" per gallon "rule" is a good baseline to follow when first stocking a tank, but unfortunately, it does not hold true for every scenario.
also, when you account for the stocking, you should be looking at the maximum size of the fish, not its current size. depending on what type of goldfish you have, youre looking at an average of 8-12" as its adult size. unless its a comet, in which case it can get much bigger than that, and realistically will need a pond for life.
the PH tester is one of the few you dont really need to use regularly. what you need to look for is the liquid master test kit. (API brand is usually the preferred kit. this will allow you to keep track of your ammonia/ nitrItes/ nitrAtes. assuming your tank was properly cycled, you should have none of the first two.
make sure its the liquid kit and not the paper test strips, as those tend to be unreliable.