Blue acara sound great, as they are a natural predator of guppies. Just note that they have a wild diet that often comprises of predominantly plant matter. Not that they won't eat small fish, but I wouldn't depend on the possibility of getting one that would consistently do so.
Salvini are very aggressive yes, but I wouldn't worry about them grabbing something as large as a blackskirt, although you can expect them to get chased. I'd be more worried about the catfish. I don't think there's much they can do to avoid being killed by a salvini if it turns out to be a bully.
Jack Dempseys will likely be able to swallow everything in the tank other than maybe the hoplos. If you really want a JD, get an electric blue. They're more mellow and less likely to swallow tetras and other small fish.
Salvini are very aggressive yes, but I wouldn't worry about them grabbing something as large as a blackskirt, although you can expect them to get chased. I'd be more worried about the catfish. I don't think there's much they can do to avoid being killed by a salvini if it turns out to be a bully.
Jack Dempseys will likely be able to swallow everything in the tank other than maybe the hoplos. If you really want a JD, get an electric blue. They're more mellow and less likely to swallow tetras and other small fish.