I wouldn't recommend goldfish for any beginner. One, because most of the goldfish types get big and require 30 gallons or more for just one. Second, they are huge waste producers. I'm not a huge fan of goldfish although if that's your cup of tea then go for it!!
Most fish in the trade today easily adapt to PH as long as it is constant and not at one extreme or the other. In terms of water chemistry I think that PH is of less importance than say to ammonia, nitrate or nitrites.
Well,,, My thinking has been that while pH is but a number,, It is as mentioned... a fairly Good idea of what fishes may be best able to adapt to the water I have readily available from my tap.
I would not mix fish that enjoy hard , alkaline, water such as livebearer's with fishes such as tetra's who for the most part enjoy/thrive,in more acidic Softer water.
I agree, ammonia and nitrites are of primary importance but would also submit that beginning with fishes that will do well in your particular water will be easier on the fish,,,and easier with respect to medicating/replacing fish.
The latter, becomes more like a job and far less enjoyable to many like myself.
Keep hardwater fishes in hard water,, and soft water fish in softer water and way less problems with healthy fishes IMHO.
