connecting Steel pipe to plastic OR copper

there are adapters on the shelves at lowe's hardware. there is older hard cpvc that glues together; now there's some soft new flexible hose stuff too. however: once you start opening the old galvanized pipe, you will see how restricted the water flow is inside and want to replace it. i did this is to bring the arriving fast water at the basement to the rest of the slow faucets upstairs. regardless of the expert advice of others, i use a minimum of 3/4" inside diameter water lines throughout the home. not 1/2". this will provide improved flow even in a low water pressure area [like my 42 psi.] you must have fast water arriving in the main at the basement for all this to work, of course.