you should NOT get a fish for the cycle ! put a peice of raw shrimp and put it in to decay and start ur cycle. using a fish will most likly kill it, the amonia and nitrate damage its gills and burn off its slime coat, its pretty much animal abuse and ur LFS will tell you to buy one just to make money off of u cuz they no the chance of u having to buy another fish after the one u cycle with dies. the key to this hobby is patience, especially in the beginning. its very tough to look at a tank with nuthing in it for a few weeks but its the safest and most cost effective way.
good luck and take ur time
as to the rest of ur questions:
when you do get a fish feed it a varity of foods
lighting should be 8 hours a day
green stuff comming from rock-?? pic would help
centipede = bristleworm most likly, these are good to have in ur tank this is what they look like
https://forums.saltwaterfish.com/t/314761/worm-bug-id-please
theres lots of cool things that will come out of ur rock eventually keep an eye out