I don't know how much more you can get away with, since your tank is small and you are looking for peaceful inverts. As far as inverts that are offered for sale, only a few pop to mind. I may not have scientific names for you, however -- I'm in the process of moving and my books are all over the place.
What about a yellow sea cucumber (Colochirus robustus)? They are thought to be a safe "cuke," are bright yellow in color, and are rather small (I rarely see them over an inch in length.)
Another animal you could try is a squat lobster. They are rather small and usually live on a host animal, such as an echinoderm, but I know of someone who is having success with them without a host. They are very interesting looking animals.
I have always wanted a few bubble snails for one of my tanks, but I have not been able to locate any. Some feed on plychaete worms while others are herbivores. They are beautiful animals.
If you wanted another type of animal, and were interested in an urchin, I would suggest the tuxedo urchin, Mespilia globulus. They remain relatively small (under 3" in diameter) and you can count on them to be a reliable herbivore. They will camouflage themselves with whatever their tube feet can pick up, though.
The other animals that I can think of aren't unusual as much as they are under-appreciated. Lots of them aren't even for sale. They are the sorts of animals that I keep my eyes open for at the LFS to see if they will throw them in when I make a purchase.
There is a (maybe more) species of "feather" sea cucumber (sometimes called Medusa worms) from FL that sometimes hitchhikes in on algae that is interesting to watch. They don't get very large and they are usually orangish or yellowish, if memory serves.
Some of the micro and mini brittle stars are extremely beautiful, with nice patterns and vivid colors. When I notice these in tanks, I ask for them. I love watching these little scavengers.
Another animal that I enjoy keeping are the hitchhiking bivalves that are often found on live rock. One of my LFS will pull live rock out of tank if it is covered in algae, and let it dry to kill the algae. I'll go in and look at those rocks, even if they appear to be completely dry, and pull all of the bivalves that I can find off of them. Sometimes I am too late and the animal has died, but I have rescued a number of hitchhikers this way. And those bivalves are now performing free filtering services in my tanks.
In fact, the few times that I have bought actual pieces of coral from a LFS (rather than frags from a fellow reefer), it has been because off the hitchhikers associated with the coral as much as the coral itself. I once bought a mushroom "rock" that was really a number of oysters that had attached to one another. It also included a very large barnacle. Ultimately, I was only able to keep those guys alive for a year, but I am hopeful that it was better than they would have done in the store. And their shells have become one of my ring cowrie's favorite places to feed, so that is a plus (for me and the cowrie, at least.)
