The facts:
common plecos (most people seem to buy this pleco instead of the smaller spcies like bristlenose) will grow to 2 feet in length. Unless you want to stunt the fish, wich causes spine deformity and messes up organ develpment (should be very painfull), you will need at least a 300-500g tank, and that's being conservitive.
bala sharks grow to be 1 foot long, and will probably present a risk of breaking a 50g tank when the start to get larger (a 6-8 inch bala would probably be capable of doing the job). Unless you want them to get stunted, or break your tank, you will need atleast a 125g, and, as said in the above, that's being conservative.
The tetras, "x ray catfish" and tiger barbs need/shoul have larger school sizes. I suggest you double thier populations (atleast 6 of each) to give them a proper school. Unlike the above fishes, these fish present no risk of outgrowing or breaking your tank.
Also, if you want to know more, visit my member page for links - there's tons of them there.