I just got my big tank set up today for my Jardini, so I'm probably just as excited as you are!
Anyhow, my arowana is about a foot or so long. Jardini's can be rough, mine has pretty much kicked the snot out of anything I try to put with him (except an endli. bichir and a big pleco). I've had a redtail catfish with him, but he doesn't like the RTC anymore so I'm trying to sell it now. Also, after my tank is finished conditioning, I'm going to try to introduce some datnoids the day I move the Jardini into the bigger tank. But we'll have to see how that works out.
Oops, sorry for rambling. But essentially, it's really up to the arowana. Jardini Arowanas seem have highly developed personalities, so deciding on tankmates is pretty much a flip of a coin. You really just have to see if he'll get along with them. Also, Jardinis tend to get more agressive once they're almost half grown, so keep that in mind also.
As for silvers, well they can pretty much get along well in any monster community tank. They'd be the "safe bet" but not nearly as cool in my opinion.
P.s.
Make sure none of your fish can fit into the other fish's mouth, that's important either way. So nothing that can eat your jardini, and nothing your jardini can eat.