hi marea,
1]yes they can and they are aggressive but they school very well. max 6 in 3ft tank. eats anything...pellets, shrimp...
2]non iodized salt is fine, marine salt is better.
3]good question...brackish water means it's anywhere between river water and sea water. sea water has 35g /liter of salinity. somewhere in between should be fine. so figure out how much water you got in your tank in liters. you'd have to do this in step every hour up the salinity 100g per hour or so till you reach around 10-15g/liter because dumping all the salt in at once will shock the fish.
for new fish, drip slowly your aq water into the bags of new fish before netting them to transfer into your tank.
4]no you don't have to. they prefer aragonite sand or gravel.
http://64.73.41.105/product/prod_display.cfm?pcatid=967
IME crushed coral is fine too. this raises the ph which they like.
5]nemos live in seawater, so you need to prepare for a marine setup if you want to put nemos in.
seawater doesn't change much because of the huge body of water[buffer] hence 'nemos' do not tolerate changes as well as freshwater fish.
so don't put nemos into brackish water, that will be dire...
er...your sabae's will have nemos for lunch even if it kills them...
lastly, you probably need to take a peek at the marine section for more answers...especially filtration, stocking levels before you take another step...