what format must video clips be in to be accepted
Impress doesn’t know how to play video clips. It uses the capability of the underlying system.
On Linux it uses the gstreamer module. The accepted formats depend on the installed gstreamer plugins. If your media format is not played in Impress install extra gstreamer plugins.
Some Linux distros (like openSuSE) supply a version of gsteamer that is crippled for legal reasons. Installing gstreamer from a proper multimedia repository (like Packman for openSuSE) will usually solve the problem, but may not be legal in your country. Gsteamer plugins may be legally obtained from a company called Fluendo.
On Windows it uses a windows multimedia framework. If a media format is not played in Impress try to install extra codecs. Beware that installing codec packs like the K-lite codec pack usually does the trick, but may not be legal in your country.
To be more precise: On Windows it is using the DirectShow type of plugins/codecs - and on Mac it uses Mac’s way (and that is via quicktime)