the general rule with arowanas is, don't keep them with anything they could fit in their mouth.. silver arowanas aren't highly aggressive, and with enough space don't tend to pick fights with their tank mates... small arowanas do grow fast though because they're pretty much always hungry.. your growing arowana could eventually see them as food.. probably within a month or two and maybe sooner... since the cory's have the spines your arowanas could choke on them..
my silver arowana grew from 7" to 24" in a year, with daily feeding of a highly varied diet.. people say you can regulate their food and "slow" their growth, but that's not a solution obviously..
as far as temperament goes, my arowana is VERY chill.. she rarely bothers anyone else in the tank (she lives with a big pleco, a large oscar, a large jack dempsey, and a large green terror).. recently we added a 7" jardini arowana to the mix, and they actually get along fine.. i don't keep discus, but i've heard their fragile, if they're large enough they could be ok.. maybe someone else will weigh in on that...
the silver arowana is less aggressive than the jardini arowana... black arowanas are also available but they tend to be a lot more expensive!
to address your idea of having one OR two arowanas, anyone would tell you having two in a tank is risk.. we did it because we knew our arowana was very relaxed and not very territorial, and the jardini was small enough that it worked (they tend to be more on the aggressive side)...
now.. if you're planning on getting two arowanas that are equal in size and raising them together that is your best bet for having them get along... the size of your tank will pose a problem in the long run... to house one or even two arowanas you need at least a 300g tank... they need lots of room to swim!
so in your decision to purchase an arowana, you need to plan for expanding your aquarium set up in less than 3 years... we'll keep our 180 for another year or so, but will definitely have to move our arowanas over to a larger tank once our jardini gets to be full grown and the silver is full grown (they can get to up to 3 feet in some cases)...
hope that helps!
-kyra
