That should be around 33 - 35g tank... I don't believe you are overstocked assuming it is a 3ft tank.
Since you already have a RTB Shark and Tiger Barbs the choices are highly limited. Since you already have some cories, perhaps you should get some more to keep them happy, and why not get a bristlenose pleco? Clown Loach will be ok in that tank as well, but they will outgrow eventually.