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.
