It's not about the number of fish in your tank. It's about the species you have and what those particular species requirements are.
For example, you should never have two red fin sharks together (I have to presume you mean sharks). Each shark will require a minimum of 55-75 gallons due to their aggression level. And yes, the sharks will eat the guppies and probably the feeders.
Feeders are know for being in horrible health, as they are not really meant to live basically. You're better off having a separate tank and raising your own feeders, if necessary.
Tiger barbs are aggressive and you should have a large school of them to curb the aggression. Guppies would not stand a chance.
Bala sharks require groups of 5 or more, as do the irridescents.
Did you research any of these species prior to purchase or did you just get really bad advice from your lfs?