Scombs only reach 12-15" in the wild as well, so I wouldn't worry about premature death.
All payara are different. Tats naturally live in cataracts in whitewater, but mine preferred still water. And some fish from slow water, like current in their aquarium. That depends on the individual fish.
Payara DO need well oxygenated water. This doesn't mean water splashing out of the aquarium from powerheads, this means that they cannot be put in small aquariums like most other aquarium fish. A 12" scomb in a 55g doesn't cut it. You will likely need a 75 for a full grown scomb.
I keep my payara in water that is 78 degrees to 82 degrees. They seem to be more prone to ick in lower temperatures, and higher temperatures just aren't that natural and deplete oxygen in the water.
Also, payara are one of the most specialized piscavores. All too many new payara keepers cannot figure out why theirs isn't taking pellets. This fish are much harder to get onto anything but live fish than just about anything else that your store likely has.
I think scombs are cool. I had two tats thinking they would be better, but they were super waspish and would fang anything that moved and were also super skittish in my presence. Scombs seem to be much more calm.
Make sure you post some pics if you get them!