using videomixer to mix multiple video streams, set the state of mainPipeline to playing using gst_element_set_state(mainPipeline, GST_STATE_PLAYING). While mainPipeline is running, modifying the properties of videomixer (adding or removing multiple sink_#) does not affect the mixing state of GST_STATE_PLAYING.
Not sure what your question or problem is exactly, but in any case you should use compositor or glvideomixer these days, the videomixer element is deprecated and has many known problems.