What experience can you share about using multiple AJAX libraries?
There are useful features in Prototype, some in jQuery, the Yahoo library, etc. Is it possible to include all libraries and use what you want from each, do they generally all play nicely together with name spaces, etc. For the sake of speed is there a practical limit to the size/number of libraries to include or is this negligible? Are there pairs that work particularly well together (e.g. Prototype/Scriptaculous) or pairs that don't?
You could use all those libraries, but I highly recommend against it. Downloading and executing that much JavaScript will most likely choke the browser and slow down your user's experience. It would be much better from a user's perspective and a developer's to pick one. Less context/architecture switching and less code to maintain.
Like other answers have said, most don't conflict.
See Yahoo!'s Exceptional Performance site for more info.