Will the Down Under Wonder work in any shower?

Yes, Extender Arms have a standard 1/2" female pipe inlet so it can be installed on any standard shower fitting. You can also use the Wonder Extender Arm with your existing shower head. And, you can also add two extenders to your current shower head to make a double wonder combo. Just remember to consider the weight of your existing head. If your shower head is heavy, either use the heavy duty "XL" Extender Arms, which are a 30% thicker, or use a lighter weight shower head. All extenders arms have a standard US 1/2" female fitting on one end and a 1/2" standard male fitting on the other end.

Do I have to hire a plumber to install it for me? 

You can install it yourself very easily because it's designed to be installed with very little actual plumbing work. The only time you need a wrench is to remove your old showerhead. The products come with simple, easy to follow installation instructions.

Deluxe Down Under Wonder Shower has a 3.25" wide head and is for use with the deluxe extension arm. The "XL" Down Under Wonder Shower has a 4.25" wide head and is for use with the XL extension arm. If you need a heavy duty unit, purchase the "XL" series.

Avoid a mismatch by ensuring you purchase an "XL" extender arm to match with your "XL" head and arm set, and that you purchase only a Deluxe extension to match with your Deluxe series head and arm set. The overall diameter of the Deluxe series and the "XL" series products are considerably different and can be readily noticed with the naked eye, even though all fittings are standard 1/2" fittings and will fit securely together.