rock first always....it will help start your cycle, as well as give a medium for bacteria to grow. You will have just rock for at least a month before you can add any corals/fish (at least in 90% of new tanks)
No, you can't just stock whatever as long as params are good. If you overcrowd you tank it will stress out the fish, even if the water quality is good. Over stressing fish can kill them, as well as make them more susceptible to parasites and diseases like ich.