This question was asked on this sub forum a couple of months ago, at that time I thought it was a good idea as well but after reading the responses and personal experience, I realized that you can probably house smaller (4-5") dats with discus (F1000, like what Oddball said) for a couple of years. I recently traded my discus out because my datnoids have become more rowdy, they steal food off of other fishes all the time and when they hit food, they hit hard! So even though a dat would probably never try to eat a discus, it can kill it easily by just trying to steal food from a discus. Just imagine a scenario where a discus is munching on a piece of shrimp, the dat tries to steal it in a classic "suction" style, misses the food but hits the discus... Discus are thin and not as strong as dats are.