Cories should have I'd say 3-4 for grouping... They poop lots :p I'm not sure I'd have 3 on each side, as they need the full space to roam - they can get scared and hurt themselves in 5 gallon space! :(
You could divide three ways... but for me I think it would be hard to have a filter and heater in there for all of them...well mainly the filter, because the center boy would get the full flow lol.
I like 2 ways :) you can add a cool hidey hole, plants, etc, easier to clean... Plus pressing the heater and filter up against the divider both get the benefit!!
I wouldn't divide 4 ways though, personally :) mainly because of the heater situation.
If you do divide, add lots of plants so they do not see each other constantly. I only have 2 who do not care to see each other, and do not get stressed. I've had many who get stressed seeing each other all the time.
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.