For me pellet training is a series of steps. But i feel you gotta have other fish in there, eating them in front of the dats, so they know its actually food.
Also it notable that pellet training is easier when small. Not too small because you might have to starve them for a little while, but around the 2.5"-4" Mark works the best for me.
Usually pellet training goes in these steps for me. Jump in where you currently are in the chain. Feeders> Shrimp> Frozen Blood Worms> Pellets. With your guys right now, I'd either stuff the shrimp with pellets to get flavor in there or soak the pellets in some water with the shrimp you're about to feed, or soak the shrimp with some frozen bloodworms. and just offer it. You need maybe a catfish to pick up the waste food that hits the bottom, but keep offering it before the food they are currently taking. Then start offering him only the target food. Eventually he will get hungry and take the target food into his mouth. Once he goes for it once, you pretty much have a done deal. Even if he spits it out for the first couple of times, he'll get hungry enough to swallow it, if you keep only offering the target food.
Oh and i saw you are feeding him freeze dried krill. I would stop offering that immediately. If you hope to get him on anything else, stop feeding that. FDK is so full of flavor that they will not easily take to anything else once hooked on it. Ive run into this problem before with one of my dats, and I kept with it because it was better than feeders, but i would say the shrimp is a better route. I never could get him on anything else. My dad raised him, and he never ate anything but FDK and eventually he died from some fungus that got into the tank. but thats besides the point. I just don't think its an adequate diet and it'll be hard to get them off of.
Oh and I usually use HBH SuperSoft krill pellets for pellet training. Has the same smell but in pellet form, and it kind of falls apart in their mouths so they take to it easier than a normal hard pellet. Then once they take to the SS i get them on either omega-one or NLS and Massivore. And then make a large fish food container mixing Algae wafers, shrimp wafers, NLS, Massivore, and omega one all together and feed that. That way they get such a broad range of food that they should no doubt have a great diet.
As far as adding more, You could try it. I don't know where or how you could have a steady supply of 3" ST or how you could afford it. Chances are they are IT, but in any case.
Adding more might spread out aggression. Most tanks with multiple Dats I've seen have a dominant great looking fish, and a bunch of not so hot looking ones. Unless they have like 10+ or are all ST.