Don't cut a hole in your pool wall and try to adapt something for an in-ground pool to put in a light - that is just asking for problems.
You can get a variety of lights that are magnetic, or can float, or people have put rope lights under the inside edge of the top rail.
There are also a number of lights that fit into your existing return. They work as both a light and a return. I have an Aqualuminator, which is one of those types. It works OK. It does not light up my entire 27' round pool, but for a smaller pool (I would say anything 20' or less) it should reach across the pool.
The Aqualimintor is OK, but it does have a bit of a weird exterior housing, and if you want to hard plumb it, there is a special adaptor you have to buy that glues on, allowing 2" PVC to connect. The part is not expensive, but just try to find a place that has them in stock (took me a while).
I guess, if you wanted more light, you could cut another return sized (2.5" or so) hole in your pool and add an other light, and just plug the pipe. I'm not sure how that would work in the winter, you might have to add a drain to get the water out, plus it would be a breeding ground for algae and other nasties.
Must return fitting lights have to come out in the winter too - or else the bulbs die. That is something else to think about.
-dave
