a pair will be fine in a 5ft tank
dont bother with plants,they will be ripped up go for a interesting rock display instead
i fed mine on a good quality pellet diet,if you have a look around some aquatic stores you may be able to purchase some older ,larger fish.I have just got a free 6ft tank in a cabinet too,complete with a tiger oscar,and 2 other chiclids (not sure what yet)i had two 16" oscars and two 15" plecs in my old 5ft tank,they were fine,you will need a good filter though.