you can use your FX5 in a SW setup, but you gotta keep it clean. like a weekly breakdown and scrub up. i use a fluval 405 on my tank. i just run the sponge though, no other media. i basically use it for water movement. you could also run floss or phospate remover or something if you want. i dont in mine, but the option is there if i need it.
i realy on Live Rock for my filtration. and water changes. ive also recently been given a skimmer so thats in there too. but i ran my tank for a good while without a skimmer, with no negative effects. just keep an eye on your params.
you will want about 1-2 lbs of rock per gallon. but that amount can fluctuate. some rock is more porus than other rock, and there for lighter or heavier per cubic ft. if you can get lighter rock, get it, it has a larger surface area, and therefore more places for bacteria to live.
if you get the rock from the private seller, ask for a few handfulls of sand too. it will have all sorts of critters in it, and give your sand a 'live' boost too.