Something doesn't sound right here. The radiator cap is what allows coolant to flow to the overflow reservoir on the 94-95 Mustangs. Our radiators have a stock 16psi cap. Once the coolant reaches 16psi, the spring in the cap compresses and exposes the hole that leads to the overflow, allowing coolant to flow to the overflow tank. When the pressure drops, a vacuum is created that sucks coolant back into the radiator from the overflow tank until the pressure drops below 16psi, then the spring in the radiator cap pushes down and closes off the hole leading to the overflow tank.
Coolant venting to the overflow tank is normal. There are markings on it showing "COLD" and "HOT", showing about how much coolant should be in the reservoir when the engine is cold or hot. If you overfill the reservoir when cold, you may get coolant pushed out of the reservoir when it's hot. You can also puke a bit of coolant out of the reservoir if your car is really hot and you shut off the car. Since the car is not moving the coolant continues to heat up for a while and in some cases can overflow the reservoir. Otherwise, the only time you'll overflow the reservoir is when you have overheated the engine.
Other cars may have a different system, but this is how our Mustangs work.