Rainlendar can combine multiple calendars and view them at once, but I'm wondering whether I can add multiple calendars and selectively view them. As in, having different calendar options for each window - so my task list might show all of my calendars, while the day view only shows one calendar, and the month view shows another. If I hide a calendar, it hides it everywhere, so I suspect this isn't possible, but I thought I should ask anyway.

You did well to ask.
Using the latest beta (and several previous beta version too) you need to:
* Right click on a window
* Select "Skins" tab/page
* Click on "advanced..."
* Choose your calendar (e.g. "Shadow4 - Calendar") in the right side, on Active Skins
* Click on Options...
* In the lower side, on Window Settings, click on "..." when select "include calendars" and "exclude calendars" on Filters section.

Example:
You can set in "Shadow4 - Calendar.1" to include "Family's Events" and set in "Shadow4 - Calendar.2" to include "My Events" and "Default" or simply exclude "Family's Events" (right the opposites calendars).

That's great, thanks so much. I completely missed that option.