Yes, that's correct. If you would like to add a source without mixing it yet, you can use the BASS_MIXER_CHAN_PAUSE flag in the BASS_Mixer_StreamAddChannel call. And then remove the flag with BASS_Mixer_ChannelFlags to start mixing it.
BASS_ChannelPlay only applies to normal BASS playback channels, ie. when the BASS_STREAM_DECODE flag isn't set. So it doesn't apply to mixer sources (it'll fail with BASS_ERROR_DECODE).
