Man we need an update in the characin forum DB! But only thing you have to worry about is the knife. I don't think their mouths are very big, but ornatus are slender so if it manages to get it down past the pectoral fins you might end up with two dead fish. Black aros are the wimps of aros, least aggressive, most slender, you don't have to worry bout them and I'm surprised it lasted with the others. Jars are the meanest of aros, but with the characins you keep they aren't as capable of killing imo, bite strength (assumption here, due to skinny lower jaw) and teeth aren't as great, also not as fast as some, but it's a tough fish. I think it should be fine in the 750, if anything it'll more than likely lose to the dorados or Lucy. The gatf well you've had atf in there so I'll think that'll be fine. Assume you decided against dumping all the atf in the 750 lol probably for the best.
In short, you're only concern should be the knife now for the pims. And the 750, I'd worry for the jar, he may get comfortable and try to dominate others, and we know that won't be taken lightly.