Whatever you do, do NOT go and buy one of the pre-built CUC packages! I do not understand how people keep the number of critters alive that they suggest. To give you an idea of what was put in my 150 at the point you are at and it was enough for the inverts to survive with only minimal food being added:
5 Astraea (monsters in terms of cleaning any algae growing!)
5 Cerinths (power houses but at a slower pace in my tank)
9 hermits
1 pincushion urchin
I honestly don't think I could have put any more in the tank initially without a decent amount of them dying if I hadn't supplemented them a decent amount.
I really like the Astraea snails the only thing is they cannot get flipped upright if they end up on their shell. (I have never seen any snail flip themselves though so don't let that be the only reason you don't get them). A lot of people like the turbo snails. I had two in my 55 a few years ago and they pretty much kept the whole thing clean with only the help of a few small hermits. Just be careful to consider how much food you have available for them and build them slowly rather than throwing them all in at once.
I would not recommend a pincushion urchin, if this a brand new tank and new to the hobby. Pincushion urchins while they do eat algae, mainly munch on corraline algae. I waited over a year to let my corraline build up, before I added them to my tank. I added one of them and they are voracious eaters, wiping my tank clean in just like 3 months. If you really want an urchin in a brand new tank, be prepared to feed it some pieces of nori every few days. While its not a big deal to do.....just not sure if you want to go through the hassle right now. If you are brand new to this hobby.....I would wait a few months.
In addition... I would add some Nessarius snails that move around the sand bed to keep it stirred. The Trochous snails can right themselves if they fall, which is nice....so I would add some of them as well.
Hermits...while I love them, they will eventually search and seek out other snails for their shells, especially in a small tank. I had a Halloween hermit that even took out a skunk cleaner shrimp on me.
Skunk cleaner shrimp are nice...they will eat any left over food that your fish don't eat.
Cerinths and Astrea are good to add as well. Mexican Turbos....they are powerhouses but are like bulls in a china shop.
I would also recommend a fighting conch snail. They travel under the sand, keep it stirred and aerated, and eat detritus.
http://www.liveaquaria.com/product/prod_display.cfm?c=497+526+2174&pcatid=2174
