There are times when I want to use mootools for certain things and Prototype & script.aculo.us for others but within the same site. I've even considered adding others, but was concerned about conflicts. Anyone have experience, or am I just trying to make things too complicated for myself?
If you really, really want to do this, then you will be able to without too many problems - the main libraries are designed to behave well inside their own namespaces, with a couple of notable exceptions - from Using JQuery with Other Frameworks:
The jQuery library, and virtually all of its plugins are constrained within the jQuery namespace. As a general rule, "global" objects are stored inside the jQuery namespace as well, so you shouldn't get a clash between jQuery and any other library (like Prototype, MooTools, or YUI).
That said, there is one caveat: By default, jQuery uses "$" as a shortcut for "jQuery", which you can over-ride.
So, yes, you can do it, but you'd likely be creating maintenance headaches further down the line for yourself - subtle differences between framework functions may be obvious to you today, but come back in 6 months and it can be a whole other story! So I would recommend keeping it as simple as you can, and having as few different frameworks (preferrably 1!) as you can in your codebase.