If it were me I would go buy a 20g long and set it up just for the two zebras. Mainly because those two fish together are worth $300-$350 range just for the pair of them, so you don't want any other fish picking on them, and other bottom dwellers like the cory cats will probably out compete them for food.
Plus if by chance you lucked out even better and got a male & female then you could have yourself a really nice chunk of cash by breeding them. So IMO best thing to do is get a 20g and set it up just for them so you can tailor the tank for their needs to keep them healthy.