They are just different, not sure if they can breed together. That would be a wasteful hybrid if you ask me. Discus are 5-10 times more expensive than sevs'
Probably not,severum are a little too aggressive for a discus tank.Discus are notoriously skittish,and wouldnt do well with a fish that might beat on it.
Severums are much tougher & can withstand poorer water conditions than discus. Plan on 90% weekly water changes on discus, compared to 50% on severums. Of course, if youre changing 50% (I use a Python) than 90% isn't much more difficult.
to big of volumn water changes like that will do more harm than good except in an emergency situation. I prefere discus myself. Though they are much more sensitive to tank conditions they look a heck of allot more colorful and cool. I find sevrums for the most part bland, though they are more aggressive than discus