SubQ is the easiest and safest for most pet owners to administer fluids. The vascular system picks up the "excess fluid" at a safe rate. IV in the wrong hands can overload the cat's blood volume.
As far as whether or not the fluids go all over the body and detox, I'm guessing you're trying to help a kitty with renal issues?
If you don't know the rate in which to run the IV, don't try it. If your vet sent the cat home with SubQ then that is the only way you should administer it. The blood volume can only be increased by a certain amount and then excess fluid is removed by the kidneys. Too much blood volume via IV will not remove it faster and can kill kitty.
IV fluids are tricky for the average pet owner to administer and monitor. SubQ fluids are absorbed into the cells and capillaries and do tend to detox the animal. The IV fluids would work better and quicker, but the pet would probably have to be hospitalized and monitored 24 hours a day.
There are special medications for gall bladder related problems. You'll need to talk to your vet about that. Fluids are not designed to deal with gall bladder related problems. It is most important to find the underlying cause.
More info from you would be very helpful. Also, what kinds of tests have been done so far?