well 60x30x30 is actually 12uk gallons (15us gallons) so if they are actually the dimensions then you have a bigger tank than you thought
If cardinal tetras are going to be the only tankmates then firstly i would round them up to 6, as they prefer to be in groups of that size, then i would say you could have 4 or 5 of the "regular" cories, these include peppered, bronze (and albino), trilineatus (labelled as julli) , sterbai, schwartzi etc, some get a bit bigger than others but this doesnt matter too much.
Or, you might like to go for a larger number of the "dwarf" types of corydoras, these include hastatus and habrosus, these may be hard to find but you can have a look around on the internet if you cannot find them, again, if cardinal tetras are going to be the only tankmates then you could have, about 6-8 of these.
These just apply if cardinal tetras are going to be the only tankmates (provided you round the number of them up to 6 as suggested).