Mixing codecs and framerates can cause unexpected behaviors.
It is recommended you use videos with a frame rate which matches the frame rate set within Isadora preferences. You can convert footage to use your selected frame rate and relink the media files in the media panel if required.
Generally the framerate should be related to the refresh rate of your display, so in North America most displays refresh at 60hz making a 30fps video a great choice since it can easily be synced with the display. In Europe 50hz is a common refresh rate, so 25fps becomes preferred on this hardware.
In theory you can use many codecs in Isadora, but experience has taught us that limiting the number of codecs used in an Isadora file produces a more stable project. Hap is your best choice for any media playback that is using modified playback settings (non 1x speed, reverse, looping etc) while H264 (Mov,Mp4,wmv file wrappers) is your best choice if playing the video from start to finish at regular speed since this codec provides much smaller file sizes it is also preferred for very long duration clips.