If you showed me a dead fish that I had just bought for you two weeks ago while explaining why it didn't live, I would get the picture and ask you before I bought more fish.
You could keep the loach and build a tank around them. They are so fun to watch when you have several. If you got rid of the knife, the dragon and the pleco, you could get half a dozen (or more, I'm not sure how many fit in your size tank) of these guys and watch them race up and down the tank. They'll sleep all piled up on one another, it is funny. Mine were Larry, Leonard, and Lloyd. The names seemed appropriate for their funny, bewhiskered faces and the way they acted. They have personality; mine were very interactive. You'd still have room for top-swimmers and mid-tank fish. They'd need plenty of caves, and they really enjoy a sand bottom. Man, now I want a loach tank!
Is
this your dragon fish? If so, it needs brackish (semi-ocean) water and won't have a long life in a freshwater tank. I'd take it back.
You could keep the platies, swordtails, and maybe the betta (with dojo loaches) as long as he gets along with the platies and swords. I don't know about the bb cat, I think that when it gets bigger it will come out at night and gobble up the smaller fish.
Of course, as others have said, when you decide who to take back you should let the tank stabilize before adding other fish.
Also: Your ammonia may be 0, but I bet you're having a nitrite spike. It's okay, you're making progress, it's just kinda painful.