clean clean clean is the motto aye, i would use a mix substrate i have seen over the years that a tank full of sand could be awesome or a nightmare well if sand just sits there and the hobbyist doesnt gravel or have stir the sand so to say than a build up of noxious gases could occur and a ammonia spike could happen and wipe your fishies out, a mixture of gravel and sand would be nice and weekly water changes with gravel / sand stirring but yeah adult discus that were local raised for generations shouldnt be a problem with plants in the aquarium, go for it buddy..