i've never heard of a severum getting to be more than 10 inches let alone 16 inches like an oscar. everything i've read or seen about severums says they average around 8 inches with a few sites saying they get up to 12 inches. gt's can work but it's a gamble. it depends on the individual fish.
on a side note...if you want a JD that will be more likely to get along with other fish, why not try a blue JD. they are a bit smaller, less aggresive and in my experience just as hardy tho quite a bit more expensive.
my 2 pennies