While there may be quite a few possibilities I remember one thing I read that I thought was a bit unusual. There is a type of anenome that looks like a hydra but with more tentacles. This anenome is usually brown or a light color tan. They are nuisances between they are hard to get rid of and will spread very easily to the point of taking over your tank therefore upsetting the bacterial balance. Aptasia was the one thing about anenomes that I can remember that has the potential of having a negative affect on your tank.
You truly want to have as much live rock as possible. The living organisms in the pores have a tremendous beneficial effect with biological filtration. The rock is fairly cheap compared to other means of filtration and makes great homes for your animals. Getting as much rock as possible can only help you.
As far as the skimmer: Skimmers are primarily used in reef systems where the corals are much more sensitive to trace organics and upsets in the PH, ammonia, nitrites, nitrates, as well as many other elements and biological factors. What the skimmer does is it removes the proteins in the water before they can be manufactored into ammonia. This helps keep the water in pristine condition. Most home tank fish can live in a closed system without a skimmer with enough live rock and biological filtration with a canister or wet/dry. IMO
, although the skimmer will in no doubt help you (I am seriously saving up for one that is compatible with my wet/dry) it is not entirely necessary unless you choose to go down the route of maintaining corals and a reef setup.