my ans may not be the best... all is IMO only!
1) can add..sure... but 1 at a time lar.. do it slowly
add 1 liao..then wait for 1 week or two... test water... if everythin is alrite... then add again
2)cured and uncured... mainly is the smell.. uncured stinks lar... cured...stinks too... but not as smelly as cured...haha and cured rocks normally haf less rubbish on/in them..
3)yup yup..u can always change ur filter media as and when u like lar..
but haf to do it slowly too.... like change 1 thing..then wait for 1 week or two so tat the bacteria can settle on the new product... best is to use the old saltwater and rinse the new product..