I have kept various fish species from different continent in the same aquarium without a problem. All by using the same tap water with the same PH. As I have said, most fish species can adapt to a wide range of PH no problem at all as long as you acclimated them slowly when introduce them into their new home.
There are of course a few exceptions. It is when the fish species is very sensitive to PH, as well as your local water's PH is a little extreme.
Yes, different fish species will produce different amount of ammonia. You can control it by the amount of fish food you feed to them. After all, the true source of ammonia is what they eat. Then again, a fully cycled aquarium should be able to handle it no problem at all as long as you do not overstock your aquarium with too many fish.
I do not deny that there are some fish species can last longer under terrible conditions. It doesn't mean you should go for only these fish as a beginner in the hope of solving your problem. You either provide the ideal conditions, or you do not. In the former case, all fish will live happily after. In the latter case, it is just the matter of time for all the fish to die. To truly dealing with beginner problems, providing the ideal conditions to the fish is the best way to go. By doing so, you do not have to worry about the fish is hardy or not.
