Not a very good combination at all. The water parameters are on the very opposite side of the scales and both are aggressive! Definitely not a good investment on such expensive fish! I suggest you select one fish and try to find compatible tankmates for it!
Water parameters really mean nothing. Discus can thrive in hard water. What I'd be more worried about is the Africans and Frontasa ripping your Arowana to shreds.
Bro I have the same set up a 6 foot tank I have a 13 in aro in with my fronts and they do great just make sure that the fish u have don't fit in the arowanas mouth and you will be fine
my fronts have never bothered my aro. I wouldn't introduce a small aro either..they are too fragile for cichlids. I would let it grow 12" at least before trying to mix. a small aro could possibly get out-competed for food with ravenous cichlids.