Not sure exactly what you are going for, but the Airport Express is great for sharing internet that is restricted to 1 device, say for example at some hotels. Like, if you access limited wifi internet on your MacBook, but only to your MacBook, when you can't use that wifi with your iOS devices. I had this problem while overseas, so I bought an Airport Express because it has a handy little trick.
Connect the Airport Express to your MacBook via the ethernet cable. Access the wifi on your MacBook, then in the Settings, share the internet received over Wifi with your ethernet connection. That way, the Airport Express will stream that same wifi over its own network, so you can connect your iOS devices. It works great without any reduction in internet speed. You would still have to setup the outgoing network on your Airport Express.
I hope this helps answer one of your questions. If not, well then I typed a whole lot for nothing, haha.