1. I'd go for a sump, especially if you're willing to do a little research into the various aspects of how/why they work and design something that is going to best suit your space an application. With your planned stock you'll need good mechanical and plenty of bio filtration, so a sump seems ideal. If you're going to future-proof it even better, big is good...
2. Stocking sounds ok, I've kept black aros with peaceful SA cichlids and aggressive CA cichlids, and as long as the aro is big enough it will work fine as the cichlids are usually far more concerned about each other than the arowana. If you go for the CA cichlids, I'd suggest waiting until the aro is at least a foot long then add the cichlids when they're big enough to not be aggressive but small enough to not be a threat. I'm not sure if I'd mix an uaru with the other cichlids you suggested, they can possibly hold their own but if one of the other brutes takes a dislike to it things could get nasty. The uaru has a mouth made for eating plants and wood, where as the likes of a big jag has a mouth made for lip locking and eating fish, not to mention clearly visible teeth. Midas/red devil [and in reality most likely a hybrid of both] are around at the moment, a single one would work well with the jag/oscar/JD combo, but again might end up being a bit strong/nasty for the uaru.
Sorry I didn't reply sooner, I saw this thread a couple of times while browsing on my phone and decided to wait til I got home to write a decent reply, then forgot....
:facepalm: