They are not really aggressive as far as going out of their way to attack tankmates. They are predatory however and will eat small fish. I dont think you have to keep them in a species tank. You can probably keep an indo dat with them because both of these fish dont grow very fast. Growth is actually pretty slow for scombs IME. Hope this helps
There shouldnt be any aggression between the two but the scombs are way more skittish then the dats. Just make sure the scombs are getting enough to eat cause the dats might beat them to it. Hope this helps!