I would not combine multiple window families into one family to accomplish this. It'd get pretty messy. Certain elements would likely be duplicated, and you'd have to associate a bunch of parameters to the new family. You CAN use existing families as roadmaps to understand how to build the new one, or you can pull elements from other families into the new one, but again, make sure you understand how to associate any parameters or constrain any geometry.
Most of what I learned about family creation came from reading this forum and YouTube. Go to YouTube for the general idea, and then post or search here for specific questions you have. Windows and doors can be some of the more complicated families to make, so that's kind of a tough one to jump into. But they also provide a lot of opportunity to learn about general family creation that will carry over into a lot of other things you can make.
If you are in a real pinch, you can probably find something like that on BIMobject.com. This is usually a last resort for me, because a lot of those families are bloated with excess parameters or frankly don't function very well, but they can bail you out if you don't have time for anything else (assuming you can find what you're looking for).
