the temp won't bother the discus much. I keep my adults at 84. the younger ones will benefit from the higher temp's
you have some that are not yet mature.I would keep the temp in the mid to upper 80's.
the fluttering could be signs of dominance. is this particular discus flaring the fins? are there any that keep their fins down in the presence of this one?
I wouldn't think too much of it but keep an eye on the fish.
one thing..the cory's are new additions? yes? did you qt the cory's prior to adding them to the tank? the cory's may prefer cooler temps.
the oto's may be okay.
also..oto's (some ) have a tendency to suck slime off discus..watch for that.
the discus could be showing signs of irritations.