Scissortail dartfish are just like all the other dartfish, M/F, M/F/F (maybe, not 100% sure on this), or single only. They do not school. As you probably know, good luck sexing them. We got three scissortail dartfish and well the third one was constantly picked on and finally found a gap in the lid after smashing his head into the glass lid at least a dozen times over the course of a year. That thing must have had brain damage. I suspect we ended up purchasing M/M/F as it is impossible to tell the genders unless you are the fish! The remaining pair we have had for ~18 months. Something I did not know about the scissortail dartfish is that they spent most of their time, i.e. all of the time except when they think it is dinner time, in their hidey hole.
I think in a tank your size you could get away with a firefish pair and a scissortail dartfish pair. You might be able to get away with a yasha goby and another non-sandsifting goby too (but I have not had any experience mixing gobies, I have just stuck to one per tank). Many people like the sand-sifting goby, but I have not had positive experiences with them like the diamond watchman goby. They throw sand everywhere potentially killing corals and may have trouble getting enough nutrition even in a tank your size. Also if you did not glue your aquascaping together and/or put your rock in on the bottom of the tank before you added sand, these guys will cause rock collapses. All of these things and more were my experience with sand-sifting gobies.