Halfbeaks should be fine, but scats need brackish water and end up moving into saltwater as they become adults.
I've had my halfbeak for over a year now in freshwater and he is still thriving and biting at my fingers. Many pet stores claim that halfbeaks need brackish water, however the amount of salt they add even the most sensitive softwater fishes could tolerate. Now on the other hand, if you don't do water changes and your pH drops, halfbeaks would probably be one of the first fish to go.
I don't think your halfbeaks would eat your scarlet badis.
scats must have brack and half beaks will really only do well with some salt to
Halfbeaks being found occasionally in brackish waters, would use marine salt if they needed it. Aquarium salt couldn't hurt but in my opinion isn't necessary. Some people like to use it, I guess it could help ward off parasites.
With your aquarium I would not add marine salt. It is not good for freshwater fish once you start trying to make an aquarium brackish.
Also, if you get halfbeaks, try to get as few as possible. They fight really easily and in my experience most halfbeaks tend to be males, and even little halfbeaks that look like females can still be males that haven't developed their male colors yet.
Any more than 1 will fight. If there are a few, the weakest ones will continually get picked off if there isn't enough room. I have two in my 40 gallon tank and they seem to get along, but I also have a lot of floating plants to break the lines of sight. If you have floating plants, you might be able to get away with up to 4, but one is always best.
Halfbeaks will jump. I would make sure you have a closed lid at night. Mine never jumped out the back, but without a lid it's bound to happen.
As for the sabertooth barracudas, scombs are generally bullet shade gray, rhaphs are bright silver, armatus are expensive (at least 60 dollars), and red tails have red tails.