What happens when you put IRON PIPE together with COPPER PIPE and use this for water supply?
What if this system is used for a swimming pool with a salt type softener system?

This is actually a question regarding chemistry, not physics.

When you connect dissimilar metals, you can set up a galvanic cell. This can lead to extensive corrosion (dissolution of the iron at the point of contact) and failure of the connection. Dielectric (insulating) unions are used to connect the two pipes in order to prevent this from happening. Salt in the water simply makes the situation worse and accelerates the corrosion.