If you notice, it is virtually impossible to kill fishes with ammonia poisoning if you maintain a low ph of 6.5.
Although it is published that nitrifying bacteria starts dying at ph below 6, there are also studies that they continue to function even at a ph of 5. There are various strains of Nitrosonomas and Nitrospiria, a stable ph (even at low levels) allows them to grow and perform their function. Nevertheless a ph of 6 to 7 should be safe, taking into account also the fishes' requirements, some tend to be more comfortable at the lower ph range, eg SA fishes including PB, whereas others prefer a higher ph eg CAs.
As for xb, reds, the range of 6 to 7 is perfectly fine.
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