less agressive is an extremely relative term in this case...even the smallest shell dwellers are very aggresive with heterotypic fish...in a tank of that size, however, you might,
might, have success keeping other fish as long there is ample territory, though i firmly believe the corys would become quick casualtys simply because they are so small...if you want shell dwellers do research on lamprologus and neolamprologus specie as well as juliodichromis species...there are many types, and each is unique...some of the least agressive dwarf shell dwellers, however, are Lamprologus kungweensis, L signatus, L similis, L multifasciatus, and L brevis, though in a tank of that size you would not be limited to any of the dwarf species nor would you be as limited numerically as i was in my ten gallon (i keep similis). Furthermore, remember that if you plan to mix species you will not be able to keep as many as if you had only one species in the tank, don't let that deter you however, because there is certainly more visual interest in a tank with a variety of fish, even if it is limited
bear in mind that Lamprologus and Neolamprologus are often used interchangably so if there is confusion, or you can't find Neolamprologus "something" try searching under Lamprologus "something" as well
that kind of scratches the surface, if you come up with some more questions i will do my best to help, there is lots of information out there, and a few other knowledgeable members on this board as well...so ask away and you will find answers
once you get more of an idea of what you like, we can get into habitats and parameters...if you look at
http://www.cichlid-forum.com/photogallery/ the gallery is very helpful, as well as the articles and messageboard
Kevin
