Technically speaking, you do a 50% water change to remove 50% of the nitrates. So if your tank have 20ppm of Nitrates after cycling process, it should be 10 after the change.
But it depends on what you are adding back too, especially if you use NSW from bad source which might contain high amount of ANN. I heard some salt mixed do too.
The bacterias usually reside in the sand, live rocks or wool in the sump.
I ever consider using 50% salt required to start a cycle, and adjust the SG after the cycle is complete and 50% water change. I gave up the idea and use NSW for cycling instead, after I start using a algae scrubber.
Anything can give out ammonia when if they die off or excrete waste. I believe anemone do and some LPS should too.... even dead and live rocks
Its not about 1-2 at a time. I usually recommend to my friends 1-2 live stocks weekly or fortnightly depending on the waste it might generate.