I think nems are fine in a somewhat young tank, IF you have your parameters in check AND you have the right lighting. I would put nems in the difficult class, more so than most if not all coral, including sps. Stay away from the nems for now until you get used to dealing with coral.
If you are that new to the hobby, you might wanna look at the frogspawn/hammer/torch corals before buying an anemone. I have a clownfish that lives in my hammer coral. And, I must admit (though probably shouldn't!) I bought the thing THINKING it was an anemone. Learned later how green I was. (As green as the hammer, I am afraid.)
In retrospect, though - in my mind, it still offers the presence of an anemone, but lacks the locomotion! So, I am very glad I got it. I now also have a torch, and a frogspawn as well. Love the movement from them.
Mushrooms have grown very well for me too. Enough so that I have given a few back to the LFS over time. Same with GSP - can't stop it from growing, but I love the motion it adds. So, I keep it on the sand bed, and rip it in half every few months, and give half away.
Green star polyps are easy. Be sure to put them where you can keep them under control I had to remove some from the front glass on a nano the other day and it is spreading across the sand... Maybe too easy...
Yes, the GSP does grow right onto the glass, but at least in my case, when I lift it off the sand bed, the attachment to the glass peels right off too. Definitely don't put it on the rockwork, though. Unless you want a turf room tank.
star polyps grow like weeds and i sell back to fish store all the time. i have a main rock at bottom of my tank looks like grass cool to watch move pllus have it growing on the back of my tank for decora. did not want to paint back of tank so used my stars.