Looks like there's quite a bit of info. Here's some links:
Stated in the beginning there are limits to the free use of the API. This is 25000 map initialisations per day, which I consider very high.
Not sure how many overlays you can have, and hopefully control which ones are on and off using sliders and buttons. - Yep, above. "Showing and hiding Overlays"
Half way down the page on Info on Overlays theres a nice example of Animating Symbols