I've had golden gourami (same specie as your blue) pretty much kick around everything in my tank. They are sort of on the aggressive side, yours are probably young right now but when they hit adulthood, you will have a couple terrors on your hands unless you get a bigger tank and seperate them from your betta (which will probably be killed once the gourami snap).
As already mentioned, bala sharks get VERY big and spastic if they are not treated as they should be. I would take them back unless your ready to get a big tank, I would even go as far to say that a 75g cannot hold the proper sized shoal that they need... A 125g is more like it.
Most crabs, I am going to assume you have fiddler crab, need to be able to get out of the water. They are best suited in paludarium (basically a half and half tank, some where between aquarium and terrarium). He should be returned as well unless you are ready to accomodate him, possibly in his own, smaller tank. If your not careful, he will find a way out, fall off the edge of the tank, and probably die.
Really, the only fish suited for your tank is the betta. IMO two adult blue gourami need more space if they are to be kept together and they shouldn't be mixed with bettas.
Personally, I would take the balas, the gourami, and the crab back. Then I would look into smaller tetra species, like neons, cardinals, or glowlights, get about 10 of them. Then I would get 4-5 kuhli loaches or cory catfish.
HTH
EDIT: Has this tank been cycled?