Mate I'd be very careful about using products like what you mentioned. Depends on how much you value the motor...
I'd take the tank to somewhere that specialises in fuel tank repairs and cleaning.
If you don't want to go spending $ just clean the tank yourself. Amazing how well they can come up with a little elbow grease. It all depends how rooted the tank is.
It is strange to have a fuel tank that is rusted. Even water in the fuel generally makes for that white crusty shite, but not rust in the tank...