Cories would be a nice addition, but the Rainbow shark could cause problems. It may not like a group of fish taking over its territory. You could try a group of cories, but I'd get atleast 6...8 would be better. Maybe the shark wouldn't bother a group of 8 cories.
Personally, if you want more bottom feeders, I'd go with loaches instead of cories. They seem to be more aggressive (not too much though) so they'd be able to defend themselves more if the shark started picking on them. They also get a bit bigger, so they have a better chance against the shark. A group of atleast 4 loaches is best. Some options are Zebra, Yoyo, Dwarf Chain, and Polka Dot.
Also the Sailfin is going to outgrow the tank. I hope you have plans to upgrade or sell it when it gets too big for the tank. Otherwise it could be stunted. According to Planet Catfish, these guys can grow over 17 inches...which is quite large for a 55g.
http://www.planetcatfish.com/catelog/species.php?species_id=148
You have room to add another school of fish. If you want some color you could add a school of Rainbowfish. There are many types....Dwarf Neon (Praecox) rainbows, Boesemani, and Turquoise are a few options. Any schooling fish should work though, so if you wanted to add some tetras, that would be great. Since the Rosey Barbs get kind of large, I'd go with a 2nd smaller school, so tetras would be great, or the Dwarf rainbows.
