a rubbernose will quickly outgrow your tank. I agree that oto's would be enough for algae. or you can go with a bn pleco instead. they max out at about 5". I have only one in my 120 gal. and a lot of wood, and I have pretty much no algae.
cardinals are from the same waters as discus and unfortunatly there are no plants in those blackwater environments. just some floating plants such as frogbit and salvinia.
if your not concerned with a biotope then you can use many plants that fit your lighting.