I would go for 6 and be aware you will need to keep up with regular big w/c's. A pair is a good option, 4 juvies is not so good - it is 2 to few to stop damaging hierarchical squabbles from being fatal to the victims. The water does not need to be super aged as long as it is dechlored and approx the same pH/temp.
Rams are good tankmates. The pleco sucking deal is very, very rare, effectively an old wives tail. Most cories do not do well in discus tanks as they are too warm, and it is to some extent true of A. cacatuoides as well.