I was told to look at the length of the fish it add all of it up and it should be the length of the longest side of your tank. You should learn the compatibility of your fish with the others, what they eat and other basic stuff. not bad learning it the quick way by forum. But remember you need to know your fish before you buy.
they will occupy the same level of the tank, the lower levels, and this could result in terrotorial issues and the rams constantly harassing the cories. this being said i have seen lots of tanks with boths rams and cories with no problems at all.