cardinals and gobies??

That really depends on the size of your tank and the species you'd consider for each type. If you're only working with something like a 20 gallon tank, it might be better to look at smaller Goby species, though you could conceivably keep a small school in it with incredible care taken to keep water parameters in check. Purely for size reasons, smaller Gobies (Cryptocentrus cinctus, Gobiodon okinawae, Gobiodon atrangulatus, Gobiodon quinquestriatus, Elacatinus oceanops, etc.).

Yes, both are pretty hardy and do not require a specific diet.